getcpu

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



NAME
       getcpu - determine CPU and NUMA node on which the calling thread is
       running

SYNOPSIS
       #include <linux/getcpu.h>

       int getcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *tcache);

DESCRIPTION
       The getcpu() system call identifies the processor and node on which the
       calling thread or process is currently running and writes them into the
       integers pointed to by the cpu and node arguments.  The processor is a
       unique small integer identifying a CPU.  The node is a unique small
       identifier identifying a NUMA node.  When either cpu or node is NULL
       nothing is written to the respective pointer.

       The third argument to this system call is nowadays unused, and should
       be specified as NULL unless portability to Linux 2.6.23 or earlier is
       required (see NOTES).

       The information placed in cpu is guaranteed to be current only at the
       time of the call: unless the CPU affinity has been fixed using
       sched_setaffinity(2), the kernel might change the CPU at any time.
       (Normally this does not happen because the scheduler tries to minimize
       movements between CPUs to keep caches hot, but it is possible.)  The
       caller must allow for the possibility that the information returned in
       cpu and node is no longer current by the time the call returns.

RETURN VALUE
       On success, 0 is returned.  On error, -1 is returned, and errno is set
       appropriately.

ERRORS
       EFAULT Arguments point outside the calling process's address space.

VERSIONS
       getcpu() was added in kernel 2.6.19 for x86-64 and i386.  Library
       support was added in glibc 2.29 (Earlier glibc versions did not provide
       a wrapper for this system call, necessitating the use of syscall(2).)

CONFORMING TO
       getcpu() is Linux-specific.

NOTES
       Linux makes a best effort to make this call as fast as possible.  (On
       some architectures, this is done via an implementation in the vdso(7).)
       The intention of getcpu() is to allow programs to make optimizations
       with per-CPU data or for NUMA optimization.

       The tcache argument is unused since Linux 2.6.24.  In earlier kernels,
       if this argument was non-NULL, then it specified a pointer to a caller-
       allocated buffer in thread-local storage that was used to provide a
       caching mechanism for getcpu().  Use of the cache could speed getcpu()
       calls, at the cost that there was a very small chance that the returned
       information would be out of date.  The caching mechanism was considered
       to cause problems when migrating threads between CPUs, and so the
       argument is now ignored.

SEE ALSO
       mbind(2), sched_setaffinity(2), set_mempolicy(2), sched_getcpu(3),
       cpuset(7), vdso(7)

COLOPHON
       This page is part of release 5.04 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                             2019-03-06                         GETCPU(2)