perf_event_open

PERF_EVENT_OPEN(2)          Linux Programmer's Manual         PERF_EVENT_OPEN(2)



NAME
       perf_event_open - set up performance monitoring

SYNOPSIS
       #include <linux/perf_event.h>
       #include <linux/hw_breakpoint.h>

       int perf_event_open(struct perf_event_attr *attr,
                           pid_t pid, int cpu, int group_fd,
                           unsigned long flags);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION
       Given a list of parameters, perf_event_open() returns a file descriptor,
       for use in subsequent system calls (read(2), mmap(2), prctl(2), fcntl(2),
       etc.).

       A call to perf_event_open() creates a file descriptor that allows
       measuring performance information.  Each file descriptor corresponds to
       one event that is measured; these can be grouped together to measure
       multiple events simultaneously.

       Events can be enabled and disabled in two ways: via ioctl(2) and via
       prctl(2).  When an event is disabled it does not count or generate
       overflows but does continue to exist and maintain its count value.

       Events come in two flavors: counting and sampled.  A counting event is
       one that is used for counting the aggregate number of events that occur.
       In general, counting event results are gathered with a read(2) call.  A
       sampling event periodically writes measurements to a buffer that can then
       be accessed via mmap(2).

   Arguments
       The pid and cpu arguments allow specifying which process and CPU to
       monitor:

       pid == 0 and cpu == -1
              This measures the calling process/thread on any CPU.

       pid == 0 and cpu >= 0
              This measures the calling process/thread only when running on the
              specified CPU.

       pid > 0 and cpu == -1
              This measures the specified process/thread on any CPU.

       pid > 0 and cpu >= 0
              This measures the specified process/thread only when running on
              the specified CPU.

       pid == -1 and cpu >= 0
              This measures all processes/threads on the specified CPU.  This
              requires CAP_PERFMON (since Linux 5.8) or CAP_SYS_ADMIN capability
              or a /proc/sys/kernel/perf_event_paranoid value of less than 1.

       pid == -1 and cpu == -1
              This setting is invalid and will return an error.

       When pid is greater than zero, permission to perform this system call is
       governed by CAP_PERFMON (since Linux 5.9) and a ptrace access mode
       PTRACE_MODE_READ_REALCREDS check on older Linux versions; see ptrace(2).

       The group_fd argument allows event groups to be created.  An event group
       has one event which is the group leader.  The leader is created first,
       with group_fd = -1.  The rest of the group members are created with
       subsequent perf_event_open() calls with group_fd being set to the file
       descriptor of the group leader.  (A single event on its own is created
       with group_fd = -1 and is considered to be a group with only 1 member.)
       An event group is scheduled onto the CPU as a unit: it will be put onto
       the CPU only if all of the events in the group can be put onto the CPU.
       This means that the values of the member events can be meaningfully
       compared—added, divided (to get ratios), and so on—with each other, since
       they have counted events for the same set of executed instructions.

       The flags argument is formed by ORing together zero or more of the
       following values:

       PERF_FLAG_FD_CLOEXEC (since Linux 3.14)
              This flag enables the close-on-exec flag for the created event
              file descriptor, so that the file descriptor is automatically
              closed on execve(2).  Setting the close-on-exec flags at creation
              time, rather than later with fcntl(2), avoids potential race
              conditions where the calling thread invokes perf_event_open() and
              fcntl(2) at the same time as another thread calls fork(2) then
              execve(2).

       PERF_FLAG_FD_NO_GROUP
              This flag tells the event to ignore the group_fd parameter except
              for the purpose of setting up output redirection using the
              PERF_FLAG_FD_OUTPUT flag.

       PERF_FLAG_FD_OUTPUT (broken since Linux 2.6.35)
              This flag re-routes the event's sampled output to instead be
              included in the mmap buffer of the event specified by group_fd.

       PERF_FLAG_PID_CGROUP (since Linux 2.6.39)
              This flag activates per-container system-wide monitoring.  A
              container is an abstraction that isolates a set of resources for
              finer-grained control (CPUs, memory, etc.).  In this mode, the
              event is measured only if the thread running on the monitored CPU
              belongs to the designated container (cgroup).  The cgroup is
              identified by passing a file descriptor opened on its directory in
              the cgroupfs filesystem.  For instance, if the cgroup to monitor
              is called test, then a file descriptor opened on /dev/cgroup/test
              (assuming cgroupfs is mounted on /dev/cgroup) must be passed as
              the pid parameter.  cgroup monitoring is available only for
              system-wide events and may therefore require extra permissions.

       The perf_event_attr structure provides detailed configuration information
       for the event being created.

           struct perf_event_attr {
               __u32 type;                 /* Type of event */
               __u32 size;                 /* Size of attribute structure */
               __u64 config;               /* Type-specific configuration */

               union {
                   __u64 sample_period;    /* Period of sampling */
                   __u64 sample_freq;      /* Frequency of sampling */
               };

               __u64 sample_type;  /* Specifies values included in sample */
               __u64 read_format;  /* Specifies values returned in read */

               __u64 disabled       : 1,   /* off by default */
                     inherit        : 1,   /* children inherit it */
                     pinned         : 1,   /* must always be on PMU */
                     exclusive      : 1,   /* only group on PMU */
                     exclude_user   : 1,   /* don't count user */
                     exclude_kernel : 1,   /* don't count kernel */
                     exclude_hv     : 1,   /* don't count hypervisor */
                     exclude_idle   : 1,   /* don't count when idle */
                     mmap           : 1,   /* include mmap data */
                     comm           : 1,   /* include comm data */
                     freq           : 1,   /* use freq, not period */
                     inherit_stat   : 1,   /* per task counts */
                     enable_on_exec : 1,   /* next exec enables */
                     task           : 1,   /* trace fork/exit */
                     watermark      : 1,   /* wakeup_watermark */
                     precise_ip     : 2,   /* skid constraint */
                     mmap_data      : 1,   /* non-exec mmap data */
                     sample_id_all  : 1,   /* sample_type all events */
                     exclude_host   : 1,   /* don't count in host */
                     exclude_guest  : 1,   /* don't count in guest */
                     exclude_callchain_kernel : 1,
                                           /* exclude kernel callchains */
                     exclude_callchain_user   : 1,
                                           /* exclude user callchains */
                     mmap2          :  1,  /* include mmap with inode data */
                     comm_exec      :  1,  /* flag comm events that are
                                              due to exec */
                     use_clockid    :  1,  /* use clockid for time fields */
                     context_switch :  1,  /* context switch data */

                     __reserved_1   : 37;

               union {
                   __u32 wakeup_events;    /* wakeup every n events */
                   __u32 wakeup_watermark; /* bytes before wakeup */
               };

               __u32     bp_type;          /* breakpoint type */

               union {
                   __u64 bp_addr;          /* breakpoint address */
                   __u64 kprobe_func;      /* for perf_kprobe */
                   __u64 uprobe_path;      /* for perf_uprobe */
                   __u64 config1;          /* extension of config */
               };

               union {
                   __u64 bp_len;           /* breakpoint length */
                   __u64 kprobe_addr;      /* with kprobe_func == NULL */
                   __u64 probe_offset;     /* for perf_[k,u]probe */
                   __u64 config2;          /* extension of config1 */
               };
               __u64 branch_sample_type;   /* enum perf_branch_sample_type */
               __u64 sample_regs_user;     /* user regs to dump on samples */
               __u32 sample_stack_user;    /* size of stack to dump on
                                              samples */
               __s32 clockid;              /* clock to use for time fields */
               __u64 sample_regs_intr;     /* regs to dump on samples */
               __u32 aux_watermark;        /* aux bytes before wakeup */
               __u16 sample_max_stack;     /* max frames in callchain */
               __u16 __reserved_2;         /* align to u64 */

           };

       The fields of the perf_event_attr structure are described in more detail
       below:

       type   This field specifies the overall event type.  It has one of the
              following values:

              PERF_TYPE_HARDWARE
                     This indicates one of the "generalized" hardware events
                     provided by the kernel.  See the config field definition
                     for more details.

              PERF_TYPE_SOFTWARE
                     This indicates one of the software-defined events provided
                     by the kernel (even if no hardware support is available).

              PERF_TYPE_TRACEPOINT
                     This indicates a tracepoint provided by the kernel
                     tracepoint infrastructure.

              PERF_TYPE_HW_CACHE
                     This indicates a hardware cache event.  This has a special
                     encoding, described in the config field definition.

              PERF_TYPE_RAW
                     This indicates a "raw" implementation-specific event in the
                     config field.

              PERF_TYPE_BREAKPOINT (since Linux 2.6.33)
                     This indicates a hardware breakpoint as provided by the
                     CPU.  Breakpoints can be read/write accesses to an address
                     as well as execution of an instruction address.

              dynamic PMU
                     Since Linux 2.6.38, perf_event_open() can support multiple
                     PMUs.  To enable this, a value exported by the kernel can
                     be used in the type field to indicate which PMU to use.
                     The value to use can be found in the sysfs filesystem:
                     there is a subdirectory per PMU instance under
                     /sys/bus/event_source/devices.  In each subdirectory there
                     is a type file whose content is an integer that can be used
                     in the type field.  For instance,
                     /sys/bus/event_source/devices/cpu/type contains the value
                     for the core CPU PMU, which is usually 4.

              kprobe and uprobe (since Linux 4.17)
                     These two dynamic PMUs create a kprobe/uprobe and attach it
                     to the file descriptor generated by perf_event_open.  The
                     kprobe/uprobe will be destroyed on the destruction of the
                     file descriptor.  See fields kprobe_func, uprobe_path,
                     kprobe_addr, and probe_offset for more details.

       size   The size of the perf_event_attr structure for forward/backward
              compatibility.  Set this using sizeof(struct perf_event_attr) to
              allow the kernel to see the struct size at the time of
              compilation.

              The related define PERF_ATTR_SIZE_VER0 is set to 64; this was the
              size of the first published struct.  PERF_ATTR_SIZE_VER1 is 72,
              corresponding to the addition of breakpoints in Linux 2.6.33.
              PERF_ATTR_SIZE_VER2 is 80 corresponding to the addition of branch
              sampling in Linux 3.4.  PERF_ATTR_SIZE_VER3 is 96 corresponding to
              the addition of sample_regs_user and sample_stack_user in Linux
              3.7.  PERF_ATTR_SIZE_VER4 is 104 corresponding to the addition of
              sample_regs_intr in Linux 3.19.  PERF_ATTR_SIZE_VER5 is 112
              corresponding to the addition of aux_watermark in Linux 4.1.

       config This specifies which event you want, in conjunction with the type
              field.  The config1 and config2 fields are also taken into account
              in cases where 64 bits is not enough to fully specify the event.
              The encoding of these fields are event dependent.

              There are various ways to set the config field that are dependent
              on the value of the previously described type field.  What follows
              are various possible settings for config separated out by type.

              If type is PERF_TYPE_HARDWARE, we are measuring one of the
              generalized hardware CPU events.  Not all of these are available
              on all platforms.  Set config to one of the following:

                   PERF_COUNT_HW_CPU_CYCLES
                          Total cycles.  Be wary of what happens during CPU
                          frequency scaling.

                   PERF_COUNT_HW_INSTRUCTIONS
                          Retired instructions.  Be careful, these can be
                          affected by various issues, most notably hardware
                          interrupt counts.

                   PERF_COUNT_HW_CACHE_REFERENCES
                          Cache accesses.  Usually this indicates Last Level
                          Cache accesses but this may vary depending on your
                          CPU.  This may include prefetches and coherency
                          messages; again this depends on the design of your
                          CPU.

                   PERF_COUNT_HW_CACHE_MISSES
                          Cache misses.  Usually this indicates Last Level Cache
                          misses; this is intended to be used in conjunction
                          with the PERF_COUNT_HW_CACHE_REFERENCES event to
                          calculate cache miss rates.

                   PERF_COUNT_HW_BRANCH_INSTRUCTIONS
                          Retired branch instructions.  Prior to Linux 2.6.35,
                          this used the wrong event on AMD processors.

                   PERF_COUNT_HW_BRANCH_MISSES
                          Mispredicted branch instructions.

                   PERF_COUNT_HW_BUS_CYCLES
                          Bus cycles, which can be different from total cycles.

                   PERF_COUNT_HW_STALLED_CYCLES_FRONTEND (since Linux 3.0)
                          Stalled cycles during issue.

                   PERF_COUNT_HW_STALLED_CYCLES_BACKEND (since Linux 3.0)
                          Stalled cycles during retirement.

                   PERF_COUNT_HW_REF_CPU_CYCLES (since Linux 3.3)
                          Total cycles; not affected by CPU frequency scaling.

              If type is PERF_TYPE_SOFTWARE, we are measuring software events
              provided by the kernel.  Set config to one of the following:

                   PERF_COUNT_SW_CPU_CLOCK
                          This reports the CPU clock, a high-resolution per-CPU
                          timer.

                   PERF_COUNT_SW_TASK_CLOCK
                          This reports a clock count specific to the task that
                          is running.

                   PERF_COUNT_SW_PAGE_FAULTS
                          This reports the number of page faults.

                   PERF_COUNT_SW_CONTEXT_SWITCHES
                          This counts context switches.  Until Linux 2.6.34,
                          these were all reported as user-space events, after
                          that they are reported as happening in the kernel.

                   PERF_COUNT_SW_CPU_MIGRATIONS
                          This reports the number of times the process has
                          migrated to a new CPU.

                   PERF_COUNT_SW_PAGE_FAULTS_MIN
                          This counts the number of minor page faults.  These
                          did not require disk I/O to handle.

                   PERF_COUNT_SW_PAGE_FAULTS_MAJ
                          This counts the number of major page faults.  These
                          required disk I/O to handle.

                   PERF_COUNT_SW_ALIGNMENT_FAULTS (since Linux 2.6.33)
                          This counts the number of alignment faults.  These
                          happen when unaligned memory accesses happen; the
                          kernel can handle these but it reduces performance.
                          This happens only on some architectures (never on
                          x86).

                   PERF_COUNT_SW_EMULATION_FAULTS (since Linux 2.6.33)
                          This counts the number of emulation faults.  The
                          kernel sometimes traps on unimplemented instructions
                          and emulates them for user space.  This can negatively
                          impact performance.

                   PERF_COUNT_SW_DUMMY (since Linux 3.12)
                          This is a placeholder event that counts nothing.
                          Informational sample record types such as mmap or comm
                          must be associated with an active event.  This dummy
                          event allows gathering such records without requiring
                          a counting event.

              If type is PERF_TYPE_TRACEPOINT, then we are measuring kernel
              tracepoints.  The value to use in config can be obtained from
              under debugfs tracing/events/*/*/id if ftrace is enabled in the
              kernel.

              If type is PERF_TYPE_HW_CACHE, then we are measuring a hardware
              CPU cache event.  To calculate the appropriate config value use
              the following equation:

                      (perf_hw_cache_id) | (perf_hw_cache_op_id << 8) |
                      (perf_hw_cache_op_result_id << 16)

                  where perf_hw_cache_id is one of:

                      PERF_COUNT_HW_CACHE_L1D
                             for measuring Level 1 Data Cache

                      PERF_COUNT_HW_CACHE_L1I
                             for measuring Level 1 Instruction Cache

                      PERF_COUNT_HW_CACHE_LL
                             for measuring Last-Level Cache

                      PERF_COUNT_HW_CACHE_DTLB
                             for measuring the Data TLB

                      PERF_COUNT_HW_CACHE_ITLB
                             for measuring the Instruction TLB

                      PERF_COUNT_HW_CACHE_BPU
                             for measuring the branch prediction unit

                      PERF_COUNT_HW_CACHE_NODE (since Linux 3.1)
                             for measuring local memory accesses

                  and perf_hw_cache_op_id is one of:

                      PERF_COUNT_HW_CACHE_OP_READ
                             for read accesses

                      PERF_COUNT_HW_CACHE_OP_WRITE
                             for write accesses

                      PERF_COUNT_HW_CACHE_OP_PREFETCH
                             for prefetch accesses

                  and perf_hw_cache_op_result_id is one of:

                      PERF_COUNT_HW_CACHE_RESULT_ACCESS
                             to measure accesses

                      PERF_COUNT_HW_CACHE_RESULT_MISS
                             to measure misses

              If type is PERF_TYPE_RAW, then a custom "raw" config value is
              needed.  Most CPUs support events that are not covered by the
              "generalized" events.  These are implementation defined; see your
              CPU manual (for example the Intel Volume 3B documentation or the
              AMD BIOS and Kernel Developer Guide).  The libpfm4 library can be
              used to translate from the name in the architectural manuals to
              the raw hex value perf_event_open() expects in this field.

              If type is PERF_TYPE_BREAKPOINT, then leave config set to zero.
              Its parameters are set in other places.

              If type is kprobe or uprobe, set retprobe (bit 0 of config, see
              /sys/bus/event_source/devices/[k,u]probe/format/retprobe) for
              kretprobe/uretprobe.  See fields kprobe_func, uprobe_path,
              kprobe_addr, and probe_offset for more details.

       kprobe_func, uprobe_path, kprobe_addr, and probe_offset
              These fields describe the kprobe/uprobe for dynamic PMUs kprobe
              and uprobe.  For kprobe: use kprobe_func and probe_offset, or use
              kprobe_addr and leave kprobe_func as NULL.  For uprobe: use
              uprobe_path and probe_offset.

       sample_period, sample_freq
              A "sampling" event is one that generates an overflow notification
              every N events, where N is given by sample_period.  A sampling
              event has sample_period > 0.  When an overflow occurs, requested
              data is recorded in the mmap buffer.  The sample_type field
              controls what data is recorded on each overflow.

              sample_freq can be used if you wish to use frequency rather than
              period.  In this case, you set the freq flag.  The kernel will
              adjust the sampling period to try and achieve the desired rate.
              The rate of adjustment is a timer tick.

       sample_type
              The various bits in this field specify which values to include in
              the sample.  They will be recorded in a ring-buffer, which is
              available to user space using mmap(2).  The order in which the
              values are saved in the sample are documented in the MMAP Layout
              subsection below; it is not the enum perf_event_sample_format
              order.

              PERF_SAMPLE_IP
                     Records instruction pointer.

              PERF_SAMPLE_TID
                     Records the process and thread IDs.

              PERF_SAMPLE_TIME
                     Records a timestamp.

              PERF_SAMPLE_ADDR
                     Records an address, if applicable.

              PERF_SAMPLE_READ
                     Record counter values for all events in a group, not just
                     the group leader.

              PERF_SAMPLE_CALLCHAIN
                     Records the callchain (stack backtrace).

              PERF_SAMPLE_ID
                     Records a unique ID for the opened event's group leader.

              PERF_SAMPLE_CPU
                     Records CPU number.

              PERF_SAMPLE_PERIOD
                     Records the current sampling period.

              PERF_SAMPLE_STREAM_ID
                     Records a unique ID for the opened event.  Unlike
                     PERF_SAMPLE_ID the actual ID is returned, not the group
                     leader.  This ID is the same as the one returned by
                     PERF_FORMAT_ID.

              PERF_SAMPLE_RAW
                     Records additional data, if applicable.  Usually returned
                     by tracepoint events.

              PERF_SAMPLE_BRANCH_STACK (since Linux 3.4)
                     This provides a record of recent branches, as provided by
                     CPU branch sampling hardware (such as Intel Last Branch
                     Record).  Not all hardware supports this feature.

                     See the branch_sample_type field for how to filter which
                     branches are reported.

              PERF_SAMPLE_REGS_USER (since Linux 3.7)
                     Records the current user-level CPU register state (the
                     values in the process before the kernel was called).

              PERF_SAMPLE_STACK_USER (since Linux 3.7)
                     Records the user level stack, allowing stack unwinding.

              PERF_SAMPLE_WEIGHT (since Linux 3.10)
                     Records a hardware provided weight value that expresses how
                     costly the sampled event was.  This allows the hardware to
                     highlight expensive events in a profile.

              PERF_SAMPLE_DATA_SRC (since Linux 3.10)
                     Records the data source: where in the memory hierarchy the
                     data associated with the sampled instruction came from.
                     This is available only if the underlying hardware supports
                     this feature.

              PERF_SAMPLE_IDENTIFIER (since Linux 3.12)
                     Places the SAMPLE_ID value in a fixed position in the
                     record, either at the beginning (for sample events) or at
                     the end (if a non-sample event).

                     This was necessary because a sample stream may have records
                     from various different event sources with different
                     sample_type settings.  Parsing the event stream properly
                     was not possible because the format of the record was
                     needed to find SAMPLE_ID, but the format could not be found
                     without knowing what event the sample belonged to (causing
                     a circular dependency).

                     The PERF_SAMPLE_IDENTIFIER setting makes the event stream
                     always parsable by putting SAMPLE_ID in a fixed location,
                     even though it means having duplicate SAMPLE_ID values in
                     records.

              PERF_SAMPLE_TRANSACTION (since Linux 3.13)
                     Records reasons for transactional memory abort events (for
                     example, from Intel TSX transactional memory support).

                     The precise_ip setting must be greater than 0 and a
                     transactional memory abort event must be measured or no
                     values will be recorded.  Also note that some perf_event
                     measurements, such as sampled cycle counting, may cause
                     extraneous aborts (by causing an interrupt during a
                     transaction).

              PERF_SAMPLE_REGS_INTR (since Linux 3.19)
                     Records a subset of the current CPU register state as
                     specified by sample_regs_intr.  Unlike
                     PERF_SAMPLE_REGS_USER the register values will return
                     kernel register state if the overflow happened while kernel
                     code is running.  If the CPU supports hardware sampling of
                     register state (i.e., PEBS on Intel x86) and precise_ip is
                     set higher than zero then the register values returned are
                     those captured by hardware at the time of the sampled
                     instruction's retirement.

       read_format
              This field specifies the format of the data returned by read(2) on
              a perf_event_open() file descriptor.

              PERF_FORMAT_TOTAL_TIME_ENABLED
                     Adds the 64-bit time_enabled field.  This can be used to
                     calculate estimated totals if the PMU is overcommitted and
                     multiplexing is happening.

              PERF_FORMAT_TOTAL_TIME_RUNNING
                     Adds the 64-bit time_running field.  This can be used to
                     calculate estimated totals if the PMU is overcommitted and
                     multiplexing is happening.

              PERF_FORMAT_ID
                     Adds a 64-bit unique value that corresponds to the event
                     group.

              PERF_FORMAT_GROUP
                     Allows all counter values in an event group to be read with
                     one read.

       disabled
              The disabled bit specifies whether the counter starts out disabled
              or enabled.  If disabled, the event can later be enabled by
              ioctl(2), prctl(2), or enable_on_exec.

              When creating an event group, typically the group leader is
              initialized with disabled set to 1 and any child events are
              initialized with disabled set to 0.  Despite disabled being 0, the
              child events will not start until the group leader is enabled.

       inherit
              The inherit bit specifies that this counter should count events of
              child tasks as well as the task specified.  This applies only to
              new children, not to any existing children at the time the counter
              is created (nor to any new children of existing children).

              Inherit does not work for some combinations of read_format values,
              such as PERF_FORMAT_GROUP.

       pinned The pinned bit specifies that the counter should always be on the
              CPU if at all possible.  It applies only to hardware counters and
              only to group leaders.  If a pinned counter cannot be put onto the
              CPU (e.g., because there are not enough hardware counters or
              because of a conflict with some other event), then the counter
              goes into an 'error' state, where reads return end-of-file (i.e.,
              read(2) returns 0) until the counter is subsequently enabled or
              disabled.

       exclusive
              The exclusive bit specifies that when this counter's group is on
              the CPU, it should be the only group using the CPU's counters.  In
              the future this may allow monitoring programs to support PMU
              features that need to run alone so that they do not disrupt other
              hardware counters.

              Note that many unexpected situations may prevent events with the
              exclusive bit set from ever running.  This includes any users
              running a system-wide measurement as well as any kernel use of the
              performance counters (including the commonly enabled NMI Watchdog
              Timer interface).

       exclude_user
              If this bit is set, the count excludes events that happen in user
              space.

       exclude_kernel
              If this bit is set, the count excludes events that happen in
              kernel space.

       exclude_hv
              If this bit is set, the count excludes events that happen in the
              hypervisor.  This is mainly for PMUs that have built-in support
              for handling this (such as POWER).  Extra support is needed for
              handling hypervisor measurements on most machines.

       exclude_idle
              If set, don't count when the CPU is running the idle task.  While
              you can currently enable this for any event type, it is ignored
              for all but software events.

       mmap   The mmap bit enables generation of PERF_RECORD_MMAP samples for
              every mmap(2) call that has PROT_EXEC set.  This allows tools to
              notice new executable code being mapped into a program (dynamic
              shared libraries for example) so that addresses can be mapped back
              to the original code.

       comm   The comm bit enables tracking of process command name as modified
              by the exec(2) and prctl(PR_SET_NAME) system calls as well as
              writing to /proc/self/comm.  If the comm_exec flag is also
              successfully set (possible since Linux 3.16), then the misc flag
              PERF_RECORD_MISC_COMM_EXEC can be used to differentiate the
              exec(2) case from the others.

       freq   If this bit is set, then sample_frequency not sample_period is
              used when setting up the sampling interval.

       inherit_stat
              This bit enables saving of event counts on context switch for
              inherited tasks.  This is meaningful only if the inherit field is
              set.

       enable_on_exec
              If this bit is set, a counter is automatically enabled after a
              call to exec(2).

       task   If this bit is set, then fork/exit notifications are included in
              the ring buffer.

       watermark
              If set, have an overflow notification happen when we cross the
              wakeup_watermark boundary.  Otherwise, overflow notifications
              happen after wakeup_events samples.

       precise_ip (since Linux 2.6.35)
              This controls the amount of skid.  Skid is how many instructions
              execute between an event of interest happening and the kernel
              being able to stop and record the event.  Smaller skid is better
              and allows more accurate reporting of which events correspond to
              which instructions, but hardware is often limited with how small
              this can be.

              The possible values of this field are the following:

              0  SAMPLE_IP can have arbitrary skid.

              1  SAMPLE_IP must have constant skid.

              2  SAMPLE_IP requested to have 0 skid.

              3  SAMPLE_IP must have 0 skid.  See also the description of
                 PERF_RECORD_MISC_EXACT_IP.

       mmap_data (since Linux 2.6.36)
              This is the counterpart of the mmap field.  This enables
              generation of PERF_RECORD_MMAP samples for mmap(2) calls that do
              not have PROT_EXEC set (for example data and SysV shared memory).

       sample_id_all (since Linux 2.6.38)
              If set, then TID, TIME, ID, STREAM_ID, and CPU can additionally be
              included in non-PERF_RECORD_SAMPLEs if the corresponding
              sample_type is selected.

              If PERF_SAMPLE_IDENTIFIER is specified, then an additional ID
              value is included as the last value to ease parsing the record
              stream.  This may lead to the id value appearing twice.

              The layout is described by this pseudo-structure:

                  struct sample_id {
                      { u32 pid, tid; }   /* if PERF_SAMPLE_TID set */
                      { u64 time;     }   /* if PERF_SAMPLE_TIME set */
                      { u64 id;       }   /* if PERF_SAMPLE_ID set */
                      { u64 stream_id;}   /* if PERF_SAMPLE_STREAM_ID set  */
                      { u32 cpu, res; }   /* if PERF_SAMPLE_CPU set */
                      { u64 id;       }   /* if PERF_SAMPLE_IDENTIFIER set */
                  };

       exclude_host (since Linux 3.2)
              When conducting measurements that include processes running VM
              instances (i.e., have executed a KVM_RUN ioctl(2)), only measure
              events happening inside a guest instance.  This is only meaningful
              outside the guests; this setting does not change counts gathered
              inside of a guest.  Currently, this functionality is x86 only.

       exclude_guest (since Linux 3.2)
              When conducting measurements that include processes running VM
              instances (i.e., have executed a KVM_RUN ioctl(2)), do not measure
              events happening inside guest instances.  This is only meaningful
              outside the guests; this setting does not change counts gathered
              inside of a guest.  Currently, this functionality is x86 only.

       exclude_callchain_kernel (since Linux 3.7)
              Do not include kernel callchains.

       exclude_callchain_user (since Linux 3.7)
              Do not include user callchains.

       mmap2 (since Linux 3.16)
              Generate an extended executable mmap record that contains enough
              additional information to uniquely identify shared mappings.  The
              mmap flag must also be set for this to work.

       comm_exec (since Linux 3.16)
              This is purely a feature-detection flag, it does not change kernel
              behavior.  If this flag can successfully be set, then, when comm
              is enabled, the PERF_RECORD_MISC_COMM_EXEC flag will be set in the
              misc field of a comm record header if the rename event being
              reported was caused by a call to exec(2).  This allows tools to
              distinguish between the various types of process renaming.

       use_clockid (since Linux 4.1)
              This allows selecting which internal Linux clock to use when
              generating timestamps via the clockid field.  This can make it
              easier to correlate perf sample times with timestamps generated by
              other tools.

       context_switch (since Linux 4.3)
              This enables the generation of PERF_RECORD_SWITCH records when a
              context switch occurs.  It also enables the generation of
              PERF_RECORD_SWITCH_CPU_WIDE records when sampling in CPU-wide
              mode.  This functionality is in addition to existing tracepoint
              and software events for measuring context switches.  The advantage
              of this method is that it will give full information even with
              strict perf_event_paranoid settings.

       wakeup_events, wakeup_watermark
              This union sets how many samples (wakeup_events) or bytes
              (wakeup_watermark) happen before an overflow notification happens.
              Which one is used is selected by the watermark bit flag.

              wakeup_events counts only PERF_RECORD_SAMPLE record types.  To
              receive overflow notification for all PERF_RECORD types choose
              watermark and set wakeup_watermark to 1.

              Prior to Linux 3.0, setting wakeup_events to 0 resulted in no
              overflow notifications; more recent kernels treat 0 the same as 1.

       bp_type (since Linux 2.6.33)
              This chooses the breakpoint type.  It is one of:

              HW_BREAKPOINT_EMPTY
                     No breakpoint.

              HW_BREAKPOINT_R
                     Count when we read the memory location.

              HW_BREAKPOINT_W
                     Count when we write the memory location.

              HW_BREAKPOINT_RW
                     Count when we read or write the memory location.

              HW_BREAKPOINT_X
                     Count when we execute code at the memory location.

              The values can be combined via a bitwise or, but the combination
              of HW_BREAKPOINT_R or HW_BREAKPOINT_W with HW_BREAKPOINT_X is not
              allowed.

       bp_addr (since Linux 2.6.33)
              This is the address of the breakpoint.  For execution breakpoints,
              this is the memory address of the instruction of interest; for
              read and write breakpoints, it is the memory address of the memory
              location of interest.

       config1 (since Linux 2.6.39)
              config1 is used for setting events that need an extra register or
              otherwise do not fit in the regular config field.  Raw
              OFFCORE_EVENTS on Nehalem/Westmere/SandyBridge use this field on
              Linux 3.3 and later kernels.

       bp_len (since Linux 2.6.33)
              bp_len is the length of the breakpoint being measured if type is
              PERF_TYPE_BREAKPOINT.  Options are HW_BREAKPOINT_LEN_1,
              HW_BREAKPOINT_LEN_2, HW_BREAKPOINT_LEN_4, and HW_BREAKPOINT_LEN_8.
              For an execution breakpoint, set this to sizeof(long).

       config2 (since Linux 2.6.39)
              config2 is a further extension of the config1 field.

       branch_sample_type (since Linux 3.4)
              If PERF_SAMPLE_BRANCH_STACK is enabled, then this specifies what
              branches to include in the branch record.

              The first part of the value is the privilege level, which is a
              combination of one of the values listed below.  If the user does
              not set privilege level explicitly, the kernel will use the
              event's privilege level.  Event and branch privilege levels do not
              have to match.

              PERF_SAMPLE_BRANCH_USER
                     Branch target is in user space.

              PERF_SAMPLE_BRANCH_KERNEL
                     Branch target is in kernel space.

              PERF_SAMPLE_BRANCH_HV
                     Branch target is in hypervisor.

              PERF_SAMPLE_BRANCH_PLM_ALL
                     A convenience value that is the three preceding values ORed
                     together.

              In addition to the privilege value, at least one or more of the
              following bits must be set.

              PERF_SAMPLE_BRANCH_ANY
                     Any branch type.

              PERF_SAMPLE_BRANCH_ANY_CALL
                     Any call branch (includes direct calls, indirect calls, and
                     far jumps).

              PERF_SAMPLE_BRANCH_IND_CALL
                     Indirect calls.

              PERF_SAMPLE_BRANCH_CALL (since Linux 4.4)
                     Direct calls.

              PERF_SAMPLE_BRANCH_ANY_RETURN
                     Any return branch.

              PERF_SAMPLE_BRANCH_IND_JUMP (since Linux 4.2)
                     Indirect jumps.

              PERF_SAMPLE_BRANCH_COND (since Linux 3.16)
                     Conditional branches.

              PERF_SAMPLE_BRANCH_ABORT_TX (since Linux 3.11)
                     Transactional memory aborts.

              PERF_SAMPLE_BRANCH_IN_TX (since Linux 3.11)
                     Branch in transactional memory transaction.

              PERF_SAMPLE_BRANCH_NO_TX (since Linux 3.11)
                     Branch not in transactional memory transaction.
                     PERF_SAMPLE_BRANCH_CALL_STACK (since Linux 4.1) Branch is
                     part of a hardware-generated call stack.  This requires
                     hardware support, currently only found on Intel x86 Haswell
                     or newer.

       sample_regs_user (since Linux 3.7)
              This bit mask defines the set of user CPU registers to dump on
              samples.  The layout of the register mask is architecture-specific
              and is described in the kernel header file
              arch/ARCH/include/uapi/asm/perf_regs.h.

       sample_stack_user (since Linux 3.7)
              This defines the size of the user stack to dump if
              PERF_SAMPLE_STACK_USER is specified.

       clockid (since Linux 4.1)
              If use_clockid is set, then this field selects which internal
              Linux timer to use for timestamps.  The available timers are
              defined in linux/time.h, with CLOCK_MONOTONIC,
              CLOCK_MONOTONIC_RAW, CLOCK_REALTIME, CLOCK_BOOTTIME, and CLOCK_TAI
              currently supported.

       aux_watermark (since Linux 4.1)
              This specifies how much data is required to trigger a
              PERF_RECORD_AUX sample.

       sample_max_stack (since Linux 4.8)
              When sample_type includes PERF_SAMPLE_CALLCHAIN, this field
              specifies how many stack frames to report when generating the
              callchain.

   Reading results
       Once a perf_event_open() file descriptor has been opened, the values of
       the events can be read from the file descriptor.  The values that are
       there are specified by the read_format field in the attr structure at
       open time.

       If you attempt to read into a buffer that is not big enough to hold the
       data, the error ENOSPC results.

       Here is the layout of the data returned by a read:

       * If PERF_FORMAT_GROUP was specified to allow reading all events in a
         group at once:

             struct read_format {
                 u64 nr;            /* The number of events */
                 u64 time_enabled;  /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
                 u64 time_running;  /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
                 struct {
                     u64 value;     /* The value of the event */
                     u64 id;        /* if PERF_FORMAT_ID */
                 } values[nr];
             };

       * If PERF_FORMAT_GROUP was not specified:

             struct read_format {
                 u64 value;         /* The value of the event */
                 u64 time_enabled;  /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
                 u64 time_running;  /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
                 u64 id;            /* if PERF_FORMAT_ID */
             };

       The values read are as follows:

       nr     The number of events in this file descriptor.  Available only if
              PERF_FORMAT_GROUP was specified.

       time_enabled, time_running
              Total time the event was enabled and running.  Normally these
              values are the same.  Multiplexing happens if the number of events
              is more than the number of available PMU counter slots.  In that
              case the events run only part of the time and the time_enabled and
              time running values can be used to scale an estimated value for
              the count.

       value  An unsigned 64-bit value containing the counter result.

       id     A globally unique value for this particular event; only present if
              PERF_FORMAT_ID was specified in read_format.

   MMAP layout
       When using perf_event_open() in sampled mode, asynchronous events (like
       counter overflow or PROT_EXEC mmap tracking) are logged into a ring-
       buffer.  This ring-buffer is created and accessed through mmap(2).

       The mmap size should be 1+2^n pages, where the first page is a metadata
       page (struct perf_event_mmap_page) that contains various bits of
       information such as where the ring-buffer head is.

       Before kernel 2.6.39, there is a bug that means you must allocate an mmap
       ring buffer when sampling even if you do not plan to access it.

       The structure of the first metadata mmap page is as follows:

           struct perf_event_mmap_page {
               __u32 version;        /* version number of this structure */
               __u32 compat_version; /* lowest version this is compat with */
               __u32 lock;           /* seqlock for synchronization */
               __u32 index;          /* hardware counter identifier */
               __s64 offset;         /* add to hardware counter value */
               __u64 time_enabled;   /* time event active */
               __u64 time_running;   /* time event on CPU */
               union {
                   __u64   capabilities;
                   struct {
                       __u64 cap_usr_time / cap_usr_rdpmc / cap_bit0 : 1,
                             cap_bit0_is_deprecated : 1,
                             cap_user_rdpmc         : 1,
                             cap_user_time          : 1,
                             cap_user_time_zero     : 1,
                   };
               };
               __u16 pmc_width;
               __u16 time_shift;
               __u32 time_mult;
               __u64 time_offset;
               __u64 __reserved[120];   /* Pad to 1 k */
               __u64 data_head;         /* head in the data section */
               __u64 data_tail;         /* user-space written tail */
               __u64 data_offset;       /* where the buffer starts */
               __u64 data_size;         /* data buffer size */
               __u64 aux_head;
               __u64 aux_tail;
               __u64 aux_offset;
               __u64 aux_size;

           }

       The following list describes the fields in the perf_event_mmap_page
       structure in more detail:

       version
              Version number of this structure.

       compat_version
              The lowest version this is compatible with.

       lock   A seqlock for synchronization.

       index  A unique hardware counter identifier.

       offset When using rdpmc for reads this offset value must be added to the
              one returned by rdpmc to get the current total event count.

       time_enabled
              Time the event was active.

       time_running
              Time the event was running.

       cap_usr_time / cap_usr_rdpmc / cap_bit0 (since Linux 3.4)
              There was a bug in the definition of cap_usr_time and
              cap_usr_rdpmc from Linux 3.4 until Linux 3.11.  Both bits were
              defined to point to the same location, so it was impossible to
              know if cap_usr_time or cap_usr_rdpmc were actually set.

              Starting with Linux 3.12, these are renamed to cap_bit0 and you
              should use the cap_user_time and cap_user_rdpmc fields instead.

       cap_bit0_is_deprecated (since Linux 3.12)
              If set, this bit indicates that the kernel supports the properly
              separated cap_user_time and cap_user_rdpmc bits.

              If not-set, it indicates an older kernel where cap_usr_time and
              cap_usr_rdpmc map to the same bit and thus both features should be
              used with caution.

       cap_user_rdpmc (since Linux 3.12)
              If the hardware supports user-space read of performance counters
              without syscall (this is the "rdpmc" instruction on x86), then the
              following code can be used to do a read:

                  u32 seq, time_mult, time_shift, idx, width;
                  u64 count, enabled, running;
                  u64 cyc, time_offset;

                  do {
                      seq = pc->lock;
                      barrier();
                      enabled = pc->time_enabled;
                      running = pc->time_running;

                      if (pc->cap_usr_time && enabled != running) {
                          cyc = rdtsc();
                          time_offset = pc->time_offset;
                          time_mult   = pc->time_mult;
                          time_shift  = pc->time_shift;
                      }

                      idx = pc->index;
                      count = pc->offset;

                      if (pc->cap_usr_rdpmc && idx) {
                          width = pc->pmc_width;
                          count += rdpmc(idx - 1);
                      }

                      barrier();
                  } while (pc->lock != seq);

       cap_user_time (since Linux 3.12)
              This bit indicates the hardware has a constant, nonstop timestamp
              counter (TSC on x86).

       cap_user_time_zero (since Linux 3.12)
              Indicates the presence of time_zero which allows mapping timestamp
              values to the hardware clock.

       pmc_width
              If cap_usr_rdpmc, this field provides the bit-width of the value
              read using the rdpmc or equivalent instruction.  This can be used
              to sign extend the result like:

                  pmc <<= 64 - pmc_width;
                  pmc >>= 64 - pmc_width; // signed shift right
                  count += pmc;

       time_shift, time_mult, time_offset

              If cap_usr_time, these fields can be used to compute the time
              delta since time_enabled (in nanoseconds) using rdtsc or similar.

                  u64 quot, rem;
                  u64 delta;
                  quot = (cyc >> time_shift);
                  rem = cyc & (((u64)1 << time_shift) - 1);
                  delta = time_offset + quot * time_mult +
                          ((rem * time_mult) >> time_shift);

              Where time_offset, time_mult, time_shift, and cyc are read in the
              seqcount loop described above.  This delta can then be added to
              enabled and possible running (if idx), improving the scaling:

                  enabled += delta;
                  if (idx)
                      running += delta;
                  quot = count / running;
                  rem  = count % running;
                  count = quot * enabled + (rem * enabled) / running;

       time_zero (since Linux 3.12)

              If cap_usr_time_zero is set, then the hardware clock (the TSC
              timestamp counter on x86) can be calculated from the time_zero,
              time_mult, and time_shift values:

                  time = timestamp - time_zero;
                  quot = time / time_mult;
                  rem  = time % time_mult;
                  cyc = (quot << time_shift) + (rem << time_shift) / time_mult;

              And vice versa:

                  quot = cyc >> time_shift;
                  rem  = cyc & (((u64)1 << time_shift) - 1);
                  timestamp = time_zero + quot * time_mult +
                      ((rem * time_mult) >> time_shift);

       data_head
              This points to the head of the data section.  The value
              continuously increases, it does not wrap.  The value needs to be
              manually wrapped by the size of the mmap buffer before accessing
              the samples.

              On SMP-capable platforms, after reading the data_head value, user
              space should issue an rmb().

       data_tail
              When the mapping is PROT_WRITE, the data_tail value should be
              written by user space to reflect the last read data.  In this
              case, the kernel will not overwrite unread data.

       data_offset (since Linux 4.1)
              Contains the offset of the location in the mmap buffer where perf
              sample data begins.

       data_size (since Linux 4.1)
              Contains the size of the perf sample region within the mmap
              buffer.

       aux_head, aux_tail, aux_offset, aux_size (since Linux 4.1)
              The AUX region allows mmap(2)-ing a separate sample buffer for
              high-bandwidth data streams (separate from the main perf sample
              buffer).  An example of a high-bandwidth stream is instruction
              tracing support, as is found in newer Intel processors.

              To set up an AUX area, first aux_offset needs to be set with an
              offset greater than data_offset+data_size and aux_size needs to be
              set to the desired buffer size.  The desired offset and size must
              be page aligned, and the size must be a power of two.  These
              values are then passed to mmap in order to map the AUX buffer.
              Pages in the AUX buffer are included as part of the RLIMIT_MEMLOCK
              resource limit (see setrlimit(2)), and also as part of the
              perf_event_mlock_kb allowance.

              By default, the AUX buffer will be truncated if it will not fit in
              the available space in the ring buffer.  If the AUX buffer is
              mapped as a read only buffer, then it will operate in ring buffer
              mode where old data will be overwritten by new.  In overwrite
              mode, it might not be possible to infer where the new data began,
              and it is the consumer's job to disable measurement while reading
              to avoid possible data races.

              The aux_head and aux_tail ring buffer pointers have the same
              behavior and ordering rules as the previous described data_head
              and data_tail.

       The following 2^n ring-buffer pages have the layout described below.

       If perf_event_attr.sample_id_all is set, then all event types will have
       the sample_type selected fields related to where/when (identity) an event
       took place (TID, TIME, ID, CPU, STREAM_ID) described in
       PERF_RECORD_SAMPLE below, it will be stashed just after the
       perf_event_header and the fields already present for the existing fields,
       that is, at the end of the payload.  This allows a newer perf.data file
       to be supported by older perf tools, with the new optional fields being
       ignored.

       The mmap values start with a header:

           struct perf_event_header {
               __u32   type;
               __u16   misc;
               __u16   size;
           };

       Below, we describe the perf_event_header fields in more detail.  For ease
       of reading, the fields with shorter descriptions are presented first.

       size   This indicates the size of the record.

       misc   The misc field contains additional information about the sample.

              The CPU mode can be determined from this value by masking with
              PERF_RECORD_MISC_CPUMODE_MASK and looking for one of the following
              (note these are not bit masks, only one can be set at a time):

              PERF_RECORD_MISC_CPUMODE_UNKNOWN
                     Unknown CPU mode.

              PERF_RECORD_MISC_KERNEL
                     Sample happened in the kernel.

              PERF_RECORD_MISC_USER
                     Sample happened in user code.

              PERF_RECORD_MISC_HYPERVISOR
                     Sample happened in the hypervisor.

              PERF_RECORD_MISC_GUEST_KERNEL (since Linux 2.6.35)
                     Sample happened in the guest kernel.

              PERF_RECORD_MISC_GUEST_USER  (since Linux 2.6.35)
                     Sample happened in guest user code.

              Since the following three statuses are generated by different
              record types, they alias to the same bit:

              PERF_RECORD_MISC_MMAP_DATA (since Linux 3.10)
                     This is set when the mapping is not executable; otherwise
                     the mapping is executable.

              PERF_RECORD_MISC_COMM_EXEC (since Linux 3.16)
                     This is set for a PERF_RECORD_COMM record on kernels more
                     recent than Linux 3.16 if a process name change was caused
                     by an exec(2) system call.

              PERF_RECORD_MISC_SWITCH_OUT (since Linux 4.3)
                     When a PERF_RECORD_SWITCH or PERF_RECORD_SWITCH_CPU_WIDE
                     record is generated, this bit indicates that the context
                     switch is away from the current process (instead of into
                     the current process).

              In addition, the following bits can be set:

              PERF_RECORD_MISC_EXACT_IP
                     This indicates that the content of PERF_SAMPLE_IP points to
                     the actual instruction that triggered the event.  See also
                     perf_event_attr.precise_ip.

              PERF_RECORD_MISC_EXT_RESERVED (since Linux 2.6.35)
                     This indicates there is extended data available (currently
                     not used).

              PERF_RECORD_MISC_PROC_MAP_PARSE_TIMEOUT
                     This bit is not set by the kernel.  It is reserved for the
                     user-space perf utility to indicate that /proc/i[pid]/maps
                     parsing was taking too long and was stopped, and thus the
                     mmap records may be truncated.

       type   The type value is one of the below.  The values in the
              corresponding record (that follows the header) depend on the type
              selected as shown.

              PERF_RECORD_MMAP
                  The MMAP events record the PROT_EXEC mappings so that we can
                  correlate user-space IPs to code.  They have the following
                  structure:

                      struct {
                          struct perf_event_header header;
                          u32    pid, tid;
                          u64    addr;
                          u64    len;
                          u64    pgoff;
                          char   filename[];
                      };

                  pid    is the process ID.

                  tid    is the thread ID.

                  addr   is the address of the allocated memory.  len is the
                         length of the allocated memory.  pgoff is the page
                         offset of the allocated memory.  filename is a string
                         describing the backing of the allocated memory.

              PERF_RECORD_LOST
                  This record indicates when events are lost.

                      struct {
                          struct perf_event_header header;
                          u64    id;
                          u64    lost;
                          struct sample_id sample_id;
                      };

                  id     is the unique event ID for the samples that were lost.

                  lost   is the number of events that were lost.

              PERF_RECORD_COMM
                  This record indicates a change in the process name.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                          char   comm[];
                          struct sample_id sample_id;
                      };

                  pid    is the process ID.

                  tid    is the thread ID.

                  comm   is a string containing the new name of the process.

              PERF_RECORD_EXIT
                  This record indicates a process exit event.

                      struct {
                          struct perf_event_header header;
                          u32    pid, ppid;
                          u32    tid, ptid;
                          u64    time;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_THROTTLE, PERF_RECORD_UNTHROTTLE
                  This record indicates a throttle/unthrottle event.

                      struct {
                          struct perf_event_header header;
                          u64    time;
                          u64    id;
                          u64    stream_id;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_FORK
                  This record indicates a fork event.

                      struct {
                          struct perf_event_header header;
                          u32    pid, ppid;
                          u32    tid, ptid;
                          u64    time;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_READ
                  This record indicates a read event.

                      struct {
                          struct perf_event_header header;
                          u32    pid, tid;
                          struct read_format values;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_SAMPLE
                  This record indicates a sample.

                      struct {
                          struct perf_event_header header;
                          u64    sample_id;   /* if PERF_SAMPLE_IDENTIFIER */
                          u64    ip;          /* if PERF_SAMPLE_IP */
                          u32    pid, tid;    /* if PERF_SAMPLE_TID */
                          u64    time;        /* if PERF_SAMPLE_TIME */
                          u64    addr;        /* if PERF_SAMPLE_ADDR */
                          u64    id;          /* if PERF_SAMPLE_ID */
                          u64    stream_id;   /* if PERF_SAMPLE_STREAM_ID */
                          u32    cpu, res;    /* if PERF_SAMPLE_CPU */
                          u64    period;      /* if PERF_SAMPLE_PERIOD */
                          struct read_format v;
                                              /* if PERF_SAMPLE_READ */
                          u64    nr;          /* if PERF_SAMPLE_CALLCHAIN */
                          u64    ips[nr];     /* if PERF_SAMPLE_CALLCHAIN */
                          u32    size;        /* if PERF_SAMPLE_RAW */
                          char  data[size];   /* if PERF_SAMPLE_RAW */
                          u64    bnr;         /* if PERF_SAMPLE_BRANCH_STACK */
                          struct perf_branch_entry lbr[bnr];
                                              /* if PERF_SAMPLE_BRANCH_STACK */
                          u64    abi;         /* if PERF_SAMPLE_REGS_USER */
                          u64    regs[weight(mask)];
                                              /* if PERF_SAMPLE_REGS_USER */
                          u64    size;        /* if PERF_SAMPLE_STACK_USER */
                          char   data[size];  /* if PERF_SAMPLE_STACK_USER */
                          u64    dyn_size;    /* if PERF_SAMPLE_STACK_USER &&
                                                 size != 0 */
                          u64    weight;      /* if PERF_SAMPLE_WEIGHT */
                          u64    data_src;    /* if PERF_SAMPLE_DATA_SRC */
                          u64    transaction; /* if PERF_SAMPLE_TRANSACTION */
                          u64    abi;         /* if PERF_SAMPLE_REGS_INTR */
                          u64    regs[weight(mask)];
                                              /* if PERF_SAMPLE_REGS_INTR */
                      };

                  sample_id
                      If PERF_SAMPLE_IDENTIFIER is enabled, a 64-bit unique ID
                      is included.  This is a duplication of the PERF_SAMPLE_ID
                      id value, but included at the beginning of the sample so
                      parsers can easily obtain the value.

                  ip  If PERF_SAMPLE_IP is enabled, then a 64-bit instruction
                      pointer value is included.

                  pid, tid
                      If PERF_SAMPLE_TID is enabled, then a 32-bit process ID
                      and 32-bit thread ID are included.

                  time
                      If PERF_SAMPLE_TIME is enabled, then a 64-bit timestamp is
                      included.  This is obtained via local_clock() which is a
                      hardware timestamp if available and the jiffies value if
                      not.

                  addr
                      If PERF_SAMPLE_ADDR is enabled, then a 64-bit address is
                      included.  This is usually the address of a tracepoint,
                      breakpoint, or software event; otherwise the value is 0.

                  id  If PERF_SAMPLE_ID is enabled, a 64-bit unique ID is
                      included.  If the event is a member of an event group, the
                      group leader ID is returned.  This ID is the same as the
                      one returned by PERF_FORMAT_ID.

                  stream_id
                      If PERF_SAMPLE_STREAM_ID is enabled, a 64-bit unique ID is
                      included.  Unlike PERF_SAMPLE_ID the actual ID is
                      returned, not the group leader.  This ID is the same as
                      the one returned by PERF_FORMAT_ID.

                  cpu, res
                      If PERF_SAMPLE_CPU is enabled, this is a 32-bit value
                      indicating which CPU was being used, in addition to a
                      reserved (unused) 32-bit value.

                  period
                      If PERF_SAMPLE_PERIOD is enabled, a 64-bit value
                      indicating the current sampling period is written.

                  v   If PERF_SAMPLE_READ is enabled, a structure of type
                      read_format is included which has values for all events in
                      the event group.  The values included depend on the
                      read_format value used at perf_event_open() time.

                  nr, ips[nr]
                      If PERF_SAMPLE_CALLCHAIN is enabled, then a 64-bit number
                      is included which indicates how many following 64-bit
                      instruction pointers will follow.  This is the current
                      callchain.

                  size, data[size]
                      If PERF_SAMPLE_RAW is enabled, then a 32-bit value
                      indicating size is included followed by an array of 8-bit
                      values of length size.  The values are padded with 0 to
                      have 64-bit alignment.

                      This RAW record data is opaque with respect to the ABI.
                      The ABI doesn't make any promises with respect to the
                      stability of its content, it may vary depending on event,
                      hardware, and kernel version.

                  bnr, lbr[bnr]
                      If PERF_SAMPLE_BRANCH_STACK is enabled, then a 64-bit
                      value indicating the number of records is included,
                      followed by bnr perf_branch_entry structures which each
                      include the fields:

                      from   This indicates the source instruction (may not be a
                             branch).

                      to     The branch target.

                      mispred
                             The branch target was mispredicted.

                      predicted
                             The branch target was predicted.

                      in_tx (since Linux 3.11)
                             The branch was in a transactional memory
                             transaction.

                      abort (since Linux 3.11)
                             The branch was in an aborted transactional memory
                             transaction.

                      cycles (since Linux 4.3)
                             This reports the number of cycles elapsed since the
                             previous branch stack update.

                      The entries are from most to least recent, so the first
                      entry has the most recent branch.

                      Support for mispred, predicted, and cycles is optional; if
                      not supported, those values will be 0.

                      The type of branches recorded is specified by the
                      branch_sample_type field.

                  abi, regs[weight(mask)]
                      If PERF_SAMPLE_REGS_USER is enabled, then the user CPU
                      registers are recorded.

                      The abi field is one of PERF_SAMPLE_REGS_ABI_NONE,
                      PERF_SAMPLE_REGS_ABI_32 or PERF_SAMPLE_REGS_ABI_64.

                      The regs field is an array of the CPU registers that were
                      specified by the sample_regs_user attr field.  The number
                      of values is the number of bits set in the
                      sample_regs_user bit mask.

                  size, data[size], dyn_size
                      If PERF_SAMPLE_STACK_USER is enabled, then the user stack
                      is recorded.  This can be used to generate stack
                      backtraces.  size is the size requested by the user in
                      sample_stack_user or else the maximum record size.  data
                      is the stack data (a raw dump of the memory pointed to by
                      the stack pointer at the time of sampling).  dyn_size is
                      the amount of data actually dumped (can be less than
                      size).  Note that dyn_size is omitted if size is 0.

                  weight
                      If PERF_SAMPLE_WEIGHT is enabled, then a 64-bit value
                      provided by the hardware is recorded that indicates how
                      costly the event was.  This allows expensive events to
                      stand out more clearly in profiles.

                  data_src
                      If PERF_SAMPLE_DATA_SRC is enabled, then a 64-bit value is
                      recorded that is made up of the following fields:

                      mem_op
                          Type of opcode, a bitwise combination of:

                          PERF_MEM_OP_NA          Not available
                          PERF_MEM_OP_LOAD        Load instruction
                          PERF_MEM_OP_STORE       Store instruction
                          PERF_MEM_OP_PFETCH      Prefetch
                          PERF_MEM_OP_EXEC        Executable code

                      mem_lvl
                          Memory hierarchy level hit or miss, a bitwise
                          combination of the following, shifted left by
                          PERF_MEM_LVL_SHIFT:

                          PERF_MEM_LVL_NA         Not available
                          PERF_MEM_LVL_HIT        Hit
                          PERF_MEM_LVL_MISS       Miss
                          PERF_MEM_LVL_L1         Level 1 cache
                          PERF_MEM_LVL_LFB        Line fill buffer
                          PERF_MEM_LVL_L2         Level 2 cache
                          PERF_MEM_LVL_L3         Level 3 cache
                          PERF_MEM_LVL_LOC_RAM    Local DRAM
                          PERF_MEM_LVL_REM_RAM1   Remote DRAM 1 hop
                          PERF_MEM_LVL_REM_RAM2   Remote DRAM 2 hops
                          PERF_MEM_LVL_REM_CCE1   Remote cache 1 hop
                          PERF_MEM_LVL_REM_CCE2   Remote cache 2 hops
                          PERF_MEM_LVL_IO         I/O memory
                          PERF_MEM_LVL_UNC        Uncached memory

                      mem_snoop
                          Snoop mode, a bitwise combination of the following,
                          shifted left by PERF_MEM_SNOOP_SHIFT:

                          PERF_MEM_SNOOP_NA       Not available
                          PERF_MEM_SNOOP_NONE     No snoop
                          PERF_MEM_SNOOP_HIT      Snoop hit
                          PERF_MEM_SNOOP_MISS     Snoop miss
                          PERF_MEM_SNOOP_HITM     Snoop hit modified

                      mem_lock
                          Lock instruction, a bitwise combination of the
                          following, shifted left by PERF_MEM_LOCK_SHIFT:

                          PERF_MEM_LOCK_NA        Not available
                          PERF_MEM_LOCK_LOCKED    Locked transaction

                      mem_dtlb
                          TLB access hit or miss, a bitwise combination of the
                          following, shifted left by PERF_MEM_TLB_SHIFT:

                          PERF_MEM_TLB_NA         Not available
                          PERF_MEM_TLB_HIT        Hit
                          PERF_MEM_TLB_MISS       Miss
                          PERF_MEM_TLB_L1         Level 1 TLB
                          PERF_MEM_TLB_L2         Level 2 TLB
                          PERF_MEM_TLB_WK         Hardware walker
                          PERF_MEM_TLB_OS         OS fault handler

                  transaction
                      If the PERF_SAMPLE_TRANSACTION flag is set, then a 64-bit
                      field is recorded describing the sources of any
                      transactional memory aborts.

                      The field is a bitwise combination of the following
                      values:

                      PERF_TXN_ELISION
                             Abort from an elision type transaction (Intel-CPU-
                             specific).

                      PERF_TXN_TRANSACTION
                             Abort from a generic transaction.

                      PERF_TXN_SYNC
                             Synchronous abort (related to the reported
                             instruction).

                      PERF_TXN_ASYNC
                             Asynchronous abort (not related to the reported
                             instruction).

                      PERF_TXN_RETRY
                             Retryable abort (retrying the transaction may have
                             succeeded).

                      PERF_TXN_CONFLICT
                             Abort due to memory conflicts with other threads.

                      PERF_TXN_CAPACITY_WRITE
                             Abort due to write capacity overflow.

                      PERF_TXN_CAPACITY_READ
                             Abort due to read capacity overflow.

                      In addition, a user-specified abort code can be obtained
                      from the high 32 bits of the field by shifting right by
                      PERF_TXN_ABORT_SHIFT and masking with the value
                      PERF_TXN_ABORT_MASK.

                  abi, regs[weight(mask)]
                      If PERF_SAMPLE_REGS_INTR is enabled, then the user CPU
                      registers are recorded.

                      The abi field is one of PERF_SAMPLE_REGS_ABI_NONE,
                      PERF_SAMPLE_REGS_ABI_32, or PERF_SAMPLE_REGS_ABI_64.

                      The regs field is an array of the CPU registers that were
                      specified by the sample_regs_intr attr field.  The number
                      of values is the number of bits set in the
                      sample_regs_intr bit mask.

              PERF_RECORD_MMAP2
                  This record includes extended information on mmap(2) calls
                  returning executable mappings.  The format is similar to that
                  of the PERF_RECORD_MMAP record, but includes extra values that
                  allow uniquely identifying shared mappings.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                          u64    addr;
                          u64    len;
                          u64    pgoff;
                          u32    maj;
                          u32    min;
                          u64    ino;
                          u64    ino_generation;
                          u32    prot;
                          u32    flags;
                          char   filename[];
                          struct sample_id sample_id;
                      };

                  pid    is the process ID.

                  tid    is the thread ID.

                  addr   is the address of the allocated memory.

                  len    is the length of the allocated memory.

                  pgoff  is the page offset of the allocated memory.

                  maj    is the major ID of the underlying device.

                  min    is the minor ID of the underlying device.

                  ino    is the inode number.

                  ino_generation
                         is the inode generation.

                  prot   is the protection information.

                  flags  is the flags information.

                  filename
                         is a string describing the backing of the allocated
                         memory.

              PERF_RECORD_AUX (since Linux 4.1)
                  This record reports that new data is available in the separate
                  AUX buffer region.

                      struct {
                          struct perf_event_header header;
                          u64    aux_offset;
                          u64    aux_size;
                          u64    flags;
                          struct sample_id sample_id;
                      };

                  aux_offset
                         offset in the AUX mmap region where the new data
                         begins.

                  aux_size
                         size of the data made available.

                  flags  describes the AUX update.

                         PERF_AUX_FLAG_TRUNCATED
                                if set, then the data returned was truncated to
                                fit the available buffer size.

                         PERF_AUX_FLAG_OVERWRITE
                                if set, then the data returned has overwritten
                                previous data.

              PERF_RECORD_ITRACE_START (since Linux 4.1)
                  This record indicates which process has initiated an
                  instruction trace event, allowing tools to properly correlate
                  the instruction addresses in the AUX buffer with the proper
                  executable.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                      };

                  pid    process ID of the thread starting an instruction trace.

                  tid    thread ID of the thread starting an instruction trace.

              PERF_RECORD_LOST_SAMPLES (since Linux 4.2)
                  When using hardware sampling (such as Intel PEBS) this record
                  indicates some number of samples that may have been lost.

                      struct {
                          struct perf_event_header header;
                          u64    lost;
                          struct sample_id sample_id;
                      };

                  lost   the number of potentially lost samples.

              PERF_RECORD_SWITCH (since Linux 4.3)
                  This record indicates a context switch has happened.  The
                  PERF_RECORD_MISC_SWITCH_OUT bit in the misc field indicates
                  whether it was a context switch into or away from the current
                  process.

                      struct {
                          struct perf_event_header header;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_SWITCH_CPU_WIDE (since Linux 4.3)
                  As with PERF_RECORD_SWITCH this record indicates a context
                  switch has happened, but it only occurs when sampling in CPU-
                  wide mode and provides additional information on the process
                  being switched to/from.  The PERF_RECORD_MISC_SWITCH_OUT bit
                  in the misc field indicates whether it was a context switch
                  into or away from the current process.

                      struct {
                          struct perf_event_header header;
                          u32 next_prev_pid;
                          u32 next_prev_tid;
                          struct sample_id sample_id;
                      };

                  next_prev_pid
                         The process ID of the previous (if switching in) or
                         next (if switching out) process on the CPU.

                  next_prev_tid
                         The thread ID of the previous (if switching in) or next
                         (if switching out) thread on the CPU.

   Overflow handling
       Events can be set to notify when a threshold is crossed, indicating an
       overflow.  Overflow conditions can be captured by monitoring the event
       file descriptor with poll(2), select(2), or epoll(7).  Alternatively, the
       overflow events can be captured via sa signal handler, by enabling I/O
       signaling on the file descriptor; see the discussion of the F_SETOWN and
       F_SETSIG operations in fcntl(2).

       Overflows are generated only by sampling events (sample_period must have
       a nonzero value).

       There are two ways to generate overflow notifications.

       The first is to set a wakeup_events or wakeup_watermark value that will
       trigger if a certain number of samples or bytes have been written to the
       mmap ring buffer.  In this case, POLL_IN is indicated.

       The other way is by use of the PERF_EVENT_IOC_REFRESH ioctl.  This ioctl
       adds to a counter that decrements each time the event overflows.  When
       nonzero, POLL_IN is indicated, but once the counter reaches 0 POLL_HUP is
       indicated and the underlying event is disabled.

       Refreshing an event group leader refreshes all siblings and refreshing
       with a parameter of 0 currently enables infinite refreshes; these
       behaviors are unsupported and should not be relied on.

       Starting with Linux 3.18, POLL_HUP is indicated if the event being
       monitored is attached to a different process and that process exits.

   rdpmc instruction
       Starting with Linux 3.4 on x86, you can use the rdpmc instruction to get
       low-latency reads without having to enter the kernel.  Note that using
       rdpmc is not necessarily faster than other methods for reading event
       values.

       Support for this can be detected with the cap_usr_rdpmc field in the mmap
       page; documentation on how to calculate event values can be found in that
       section.

       Originally, when rdpmc support was enabled, any process (not just ones
       with an active perf event) could use the rdpmc instruction to access the
       counters.  Starting with Linux 4.0, rdpmc support is only allowed if an
       event is currently enabled in a process's context.  To restore the old
       behavior, write the value 2 to /sys/devices/cpu/rdpmc.

   perf_event ioctl calls
       Various ioctls act on perf_event_open() file descriptors:

       PERF_EVENT_IOC_ENABLE
              This enables the individual event or event group specified by the
              file descriptor argument.

              If the PERF_IOC_FLAG_GROUP bit is set in the ioctl argument, then
              all events in a group are enabled, even if the event specified is
              not the group leader (but see BUGS).

       PERF_EVENT_IOC_DISABLE
              This disables the individual counter or event group specified by
              the file descriptor argument.

              Enabling or disabling the leader of a group enables or disables
              the entire group; that is, while the group leader is disabled,
              none of the counters in the group will count.  Enabling or
              disabling a member of a group other than the leader affects only
              that counter; disabling a non-leader stops that counter from
              counting but doesn't affect any other counter.

              If the PERF_IOC_FLAG_GROUP bit is set in the ioctl argument, then
              all events in a group are disabled, even if the event specified is
              not the group leader (but see BUGS).

       PERF_EVENT_IOC_REFRESH
              Non-inherited overflow counters can use this to enable a counter
              for a number of overflows specified by the argument, after which
              it is disabled.  Subsequent calls of this ioctl add the argument
              value to the current count.  An overflow notification with POLL_IN
              set will happen on each overflow until the count reaches 0; when
              that happens a notification with POLL_HUP set is sent and the
              event is disabled.  Using an argument of 0 is considered undefined
              behavior.

       PERF_EVENT_IOC_RESET
              Reset the event count specified by the file descriptor argument to
              zero.  This resets only the counts; there is no way to reset the
              multiplexing time_enabled or time_running values.

              If the PERF_IOC_FLAG_GROUP bit is set in the ioctl argument, then
              all events in a group are reset, even if the event specified is
              not the group leader (but see BUGS).

       PERF_EVENT_IOC_PERIOD
              This updates the overflow period for the event.

              Since Linux 3.7 (on ARM) and Linux 3.14 (all other architectures),
              the new period takes effect immediately.  On older kernels, the
              new period did not take effect until after the next overflow.

              The argument is a pointer to a 64-bit value containing the desired
              new period.

              Prior to Linux 2.6.36, this ioctl always failed due to a bug in
              the kernel.

       PERF_EVENT_IOC_SET_OUTPUT
              This tells the kernel to report event notifications to the
              specified file descriptor rather than the default one.  The file
              descriptors must all be on the same CPU.

              The argument specifies the desired file descriptor, or -1 if
              output should be ignored.

       PERF_EVENT_IOC_SET_FILTER (since Linux 2.6.33)
              This adds an ftrace filter to this event.

              The argument is a pointer to the desired ftrace filter.

       PERF_EVENT_IOC_ID (since Linux 3.12)
              This returns the event ID value for the given event file
              descriptor.

              The argument is a pointer to a 64-bit unsigned integer to hold the
              result.

       PERF_EVENT_IOC_SET_BPF (since Linux 4.1)
              This allows attaching a Berkeley Packet Filter (BPF) program to an
              existing kprobe tracepoint event.  You need CAP_PERFMON (since
              Linux 5.8) or CAP_SYS_ADMIN privileges to use this ioctl.

              The argument is a BPF program file descriptor that was created by
              a previous bpf(2) system call.

       PERF_EVENT_IOC_PAUSE_OUTPUT (since Linux 4.7)
              This allows pausing and resuming the event's ring-buffer.  A
              paused ring-buffer does not prevent generation of samples, but
              simply discards them.  The discarded samples are considered lost,
              and cause a PERF_RECORD_LOST sample to be generated when possible.
              An overflow signal may still be triggered by the discarded sample
              even though the ring-buffer remains empty.

              The argument is an unsigned 32-bit integer.  A nonzero value
              pauses the ring-buffer, while a zero value resumes the ring-
              buffer.

       PERF_EVENT_MODIFY_ATTRIBUTES (since Linux 4.17)
              This allows modifying an existing event without the overhead of
              closing and reopening a new event.  Currently this is supported
              only for breakpoint events.

              The argument is a pointer to a perf_event_attr structure
              containing the updated event settings.

       PERF_EVENT_IOC_QUERY_BPF (since Linux 4.16)
              This allows querying which Berkeley Packet Filter (BPF) programs
              are attached to an existing kprobe tracepoint.  You can only
              attach one BPF program per event, but you can have multiple events
              attached to a tracepoint.  Querying this value on one tracepoint
              event returns the id of all BPF programs in all events attached to
              the tracepoint.  You need CAP_PERFMON (since Linux 5.8) or
              CAP_SYS_ADMIN privileges to use this ioctl.

              The argument is a pointer to a structure
                  struct perf_event_query_bpf {
                      __u32    ids_len;
                      __u32    prog_cnt;
                      __u32    ids[0];
                  };

              The ids_len field indicates the number of ids that can fit in the
              provided ids array.  The prog_cnt value is filled in by the kernel
              with the number of attached BPF programs.  The ids array is filled
              with the id of each attached BPF program.  If there are more
              programs than will fit in the array, then the kernel will return
              ENOSPC and ids_len will indicate the number of program IDs that
              were successfully copied.

   Using prctl(2)
       A process can enable or disable all currently open event groups using the
       prctl(2) PR_TASK_PERF_EVENTS_ENABLE and PR_TASK_PERF_EVENTS_DISABLE
       operations.  This applies only to events created locally by the calling
       process.  This does not apply to events created by other processes
       attached to the calling process or inherited events from a parent
       process.  Only group leaders are enabled and disabled, not any other
       members of the groups.

   perf_event related configuration files
       Files in /proc/sys/kernel/

           /proc/sys/kernel/perf_event_paranoid
                  The perf_event_paranoid file can be set to restrict access to
                  the performance counters.

                  2   allow only user-space measurements (default since Linux
                      4.6).
                  1   allow both kernel and user measurements (default before
                      Linux 4.6).
                  0   allow access to CPU-specific data but not raw tracepoint
                      samples.
                  -1  no restrictions.

                  The existence of the perf_event_paranoid file is the official
                  method for determining if a kernel supports perf_event_open().

           /proc/sys/kernel/perf_event_max_sample_rate
                  This sets the maximum sample rate.  Setting this too high can
                  allow users to sample at a rate that impacts overall machine
                  performance and potentially lock up the machine.  The default
                  value is 100000 (samples per second).

           /proc/sys/kernel/perf_event_max_stack
                  This file sets the maximum depth of stack frame entries
                  reported when generating a call trace.

           /proc/sys/kernel/perf_event_mlock_kb
                  Maximum number of pages an unprivileged user can mlock(2).
                  The default is 516 (kB).

       Files in /sys/bus/event_source/devices/

           Since Linux 2.6.34, the kernel supports having multiple PMUs
           available for monitoring.  Information on how to program these PMUs
           can be found under /sys/bus/event_source/devices/.  Each subdirectory
           corresponds to a different PMU.

           /sys/bus/event_source/devices/*/type (since Linux 2.6.38)
                  This contains an integer that can be used in the type field of
                  perf_event_attr to indicate that you wish to use this PMU.

           /sys/bus/event_source/devices/cpu/rdpmc (since Linux 3.4)
                  If this file is 1, then direct user-space access to the
                  performance counter registers is allowed via the rdpmc
                  instruction.  This can be disabled by echoing 0 to the file.

                  As of Linux 4.0 the behavior has changed, so that 1 now means
                  only allow access to processes with active perf events, with 2
                  indicating the old allow-anyone-access behavior.

           /sys/bus/event_source/devices/*/format/ (since Linux 3.4)
                  This subdirectory contains information on the architecture-
                  specific subfields available for programming the various
                  config fields in the perf_event_attr struct.

                  The content of each file is the name of the config field,
                  followed by a colon, followed by a series of integer bit
                  ranges separated by commas.  For example, the file event may
                  contain the value config1:1,6-10,44 which indicates that event
                  is an attribute that occupies bits 1,6–10, and 44 of
                  perf_event_attr::config1.

           /sys/bus/event_source/devices/*/events/ (since Linux 3.4)
                  This subdirectory contains files with predefined events.  The
                  contents are strings describing the event settings expressed
                  in terms of the fields found in the previously mentioned
                  ./format/ directory.  These are not necessarily complete lists
                  of all events supported by a PMU, but usually a subset of
                  events deemed useful or interesting.

                  The content of each file is a list of attribute names
                  separated by commas.  Each entry has an optional value (either
                  hex or decimal).  If no value is specified, then it is assumed
                  to be a single-bit field with a value of 1.  An example entry
                  may look like this: event=0x2,inv,ldlat=3.

           /sys/bus/event_source/devices/*/uevent
                  This file is the standard kernel device interface for
                  injecting hotplug events.

           /sys/bus/event_source/devices/*/cpumask (since Linux 3.7)
                  The cpumask file contains a comma-separated list of integers
                  that indicate a representative CPU number for each socket
                  (package) on the motherboard.  This is needed when setting up
                  uncore or northbridge events, as those PMUs present socket-
                  wide events.

RETURN VALUE
       perf_event_open() returns the new file descriptor, or -1 if an error
       occurred (in which case, errno is set appropriately).

ERRORS
       The errors returned by perf_event_open() can be inconsistent, and may
       vary across processor architectures and performance monitoring units.

       E2BIG  Returned if the perf_event_attr size value is too small (smaller
              than PERF_ATTR_SIZE_VER0), too big (larger than the page size), or
              larger than the kernel supports and the extra bytes are not zero.
              When E2BIG is returned, the perf_event_attr size field is
              overwritten by the kernel to be the size of the structure it was
              expecting.

       EACCES Returned when the requested event requires CAP_PERFMON (since
              Linux 5.8) or CAP_SYS_ADMIN permissions (or a more permissive
              perf_event paranoid setting).  Some common cases where an
              unprivileged process may encounter this error: attaching to a
              process owned by a different user; monitoring all processes on a
              given CPU (i.e., specifying the pid argument as -1); and not
              setting exclude_kernel when the paranoid setting requires it.

       EBADF  Returned if the group_fd file descriptor is not valid, or, if
              PERF_FLAG_PID_CGROUP is set, the cgroup file descriptor in pid is
              not valid.

       EBUSY (since Linux 4.1)
              Returned if another event already has exclusive access to the PMU.

       EFAULT Returned if the attr pointer points at an invalid memory address.

       EINVAL Returned if the specified event is invalid.  There are many
              possible reasons for this.  A not-exhaustive list: sample_freq is
              higher than the maximum setting; the cpu to monitor does not
              exist; read_format is out of range; sample_type is out of range;
              the flags value is out of range; exclusive or pinned set and the
              event is not a group leader; the event config values are out of
              range or set reserved bits; the generic event selected is not
              supported; or there is not enough room to add the selected event.

       EINTR  Returned when trying to mix perf and ftrace handling for a uprobe.

       EMFILE Each opened event uses one file descriptor.  If a large number of
              events are opened, the per-process limit on the number of open
              file descriptors will be reached, and no more events can be
              created.

       ENODEV Returned when the event involves a feature not supported by the
              current CPU.

       ENOENT Returned if the type setting is not valid.  This error is also
              returned for some unsupported generic events.

       ENOSPC Prior to Linux 3.3, if there was not enough room for the event,
              ENOSPC was returned.  In Linux 3.3, this was changed to EINVAL.
              ENOSPC is still returned if you try to add more breakpoint events
              than supported by the hardware.

       ENOSYS Returned if PERF_SAMPLE_STACK_USER is set in sample_type and it is
              not supported by hardware.

       EOPNOTSUPP
              Returned if an event requiring a specific hardware feature is
              requested but there is no hardware support.  This includes
              requesting low-skid events if not supported, branch tracing if it
              is not available, sampling if no PMU interrupt is available, and
              branch stacks for software events.

       EOVERFLOW (since Linux 4.8)
              Returned if PERF_SAMPLE_CALLCHAIN is requested and
              sample_max_stack is larger than the maximum specified in
              /proc/sys/kernel/perf_event_max_stack.

       EPERM  Returned on many (but not all) architectures when an unsupported
              exclude_hv, exclude_idle, exclude_user, or exclude_kernel setting
              is specified.

              It can also happen, as with EACCES, when the requested event
              requires CAP_PERFMON (since Linux 5.8) or CAP_SYS_ADMIN
              permissions (or a more permissive perf_event paranoid setting).
              This includes setting a breakpoint on a kernel address, and (since
              Linux 3.13) setting a kernel function-trace tracepoint.

       ESRCH  Returned if attempting to attach to a process that does not exist.

VERSION
       perf_event_open() was introduced in Linux 2.6.31 but was called
       perf_counter_open().  It was renamed in Linux 2.6.32.

CONFORMING TO
       This perf_event_open() system call Linux-specific and should not be used
       in programs intended to be portable.

NOTES
       Glibc does not provide a wrapper for this system call; call it using
       syscall(2).  See the example below.

       The official way of knowing if perf_event_open() support is enabled is
       checking for the existence of the file
       /proc/sys/kernel/perf_event_paranoid.

       CAP_PERFMON capability (since Linux 5.8) provides secure approach to
       performance monitoring and observability operations in a system according
       to the principal of least privilege (POSIX IEEE 1003.1e).  Accessing
       system performance monitoring and observability operations using
       CAP_PERFMON rather than the much more powerful CAP_SYS_ADMIN excludes
       chances to misuse credentials and makes operations more secure.
       CAP_SYS_ADMIN usage for secure system performance monitoring and
       observability is discouraged in favor of the CAP_PERFMON capability.

BUGS
       The F_SETOWN_EX option to fcntl(2) is needed to properly get overflow
       signals in threads.  This was introduced in Linux 2.6.32.

       Prior to Linux 2.6.33 (at least for x86), the kernel did not check if
       events could be scheduled together until read time.  The same happens on
       all known kernels if the NMI watchdog is enabled.  This means to see if a
       given set of events works you have to perf_event_open(), start, then read
       before you know for sure you can get valid measurements.

       Prior to Linux 2.6.34, event constraints were not enforced by the kernel.
       In that case, some events would silently return "0" if the kernel
       scheduled them in an improper counter slot.

       Prior to Linux 2.6.34, there was a bug when multiplexing where the wrong
       results could be returned.

       Kernels from Linux 2.6.35 to Linux 2.6.39 can quickly crash the kernel if
       "inherit" is enabled and many threads are started.

       Prior to Linux 2.6.35, PERF_FORMAT_GROUP did not work with attached
       processes.

       There is a bug in the kernel code between Linux 2.6.36 and Linux 3.0 that
       ignores the "watermark" field and acts as if a wakeup_event was chosen if
       the union has a nonzero value in it.

       From Linux 2.6.31 to Linux 3.4, the PERF_IOC_FLAG_GROUP ioctl argument
       was broken and would repeatedly operate on the event specified rather
       than iterating across all sibling events in a group.

       From Linux 3.4 to Linux 3.11, the mmap cap_usr_rdpmc and cap_usr_time
       bits mapped to the same location.  Code should migrate to the new
       cap_user_rdpmc and cap_user_time fields instead.

       Always double-check your results!  Various generalized events have had
       wrong values.  For example, retired branches measured the wrong thing on
       AMD machines until Linux 2.6.35.

EXAMPLES
       The following is a short example that measures the total instruction
       count of a call to printf(3).

       #include <stdlib.h>
       #include <stdio.h>
       #include <unistd.h>
       #include <string.h>
       #include <sys/ioctl.h>
       #include <linux/perf_event.h>
       #include <asm/unistd.h>

       static long
       perf_event_open(struct perf_event_attr *hw_event, pid_t pid,
                       int cpu, int group_fd, unsigned long flags)
       {
           int ret;

           ret = syscall(__NR_perf_event_open, hw_event, pid, cpu,
                          group_fd, flags);
           return ret;
       }

       int
       main(int argc, char **argv)
       {
           struct perf_event_attr pe;
           long long count;
           int fd;

           memset(&pe, 0, sizeof(pe));
           pe.type = PERF_TYPE_HARDWARE;
           pe.size = sizeof(pe);
           pe.config = PERF_COUNT_HW_INSTRUCTIONS;
           pe.disabled = 1;
           pe.exclude_kernel = 1;
           pe.exclude_hv = 1;

           fd = perf_event_open(&pe, 0, -1, -1, 0);
           if (fd == -1) {
              fprintf(stderr, "Error opening leader %llx\n", pe.config);
              exit(EXIT_FAILURE);
           }

           ioctl(fd, PERF_EVENT_IOC_RESET, 0);
           ioctl(fd, PERF_EVENT_IOC_ENABLE, 0);

           printf("Measuring instruction count for this printf\n");

           ioctl(fd, PERF_EVENT_IOC_DISABLE, 0);
           read(fd, &count, sizeof(count));

           printf("Used %lld instructions\n", count);

           close(fd);
       }

SEE ALSO
       perf(1), fcntl(2), mmap(2), open(2), prctl(2), read(2)

       Documentation/admin-guide/perf-security.rst in the kernel source tree

COLOPHON
       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                              2020-11-01                 PERF_EVENT_OPEN(2)