man2/getcpu.2

   1 .\" This man page is Copyright (C) 2006 Andi Kleen <ak@muc.de>.
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  10 .\" 2008, mtk, various edits
  11 .\"
  12 .TH GETCPU 2 2019-03-06 "Linux" "Linux Programmer's Manual"
  13 .SH NAME
  14 getcpu \- determine CPU and NUMA node on which the calling thread is running
  15 .SH SYNOPSIS
  16 .nf
  17 .B #include <linux/getcpu.h>
  18 .PP
  19 .BI "int getcpu(unsigned *" cpu ", unsigned *" node \
  20 ", struct getcpu_cache *" tcache );
  21 .fi
  22 .SH DESCRIPTION
  23 The
  24 .BR getcpu ()
  25 system call identifies the processor and node on which the calling
  26 thread or process is currently running and writes them into the
  27 integers pointed to by the
  28 .I cpu
  29 and
  30 .I node
  31 arguments.
  32 The processor is a unique small integer identifying a CPU.
  33 The node is a unique small identifier identifying a NUMA node.
  34 When either
  35 .I cpu
  36 or
  37 .I node
  38 is NULL nothing is written to the respective pointer.
  39 .PP
  40 The third argument to this system call is nowadays unused,
  41 and should be specified as NULL
  42 unless portability to Linux 2.6.23 or earlier is required (see NOTES).
  43 .PP
  44 The information placed in
  45 .I cpu
  46 is guaranteed to be current only at the time of the call:
  47 unless the CPU affinity has been fixed using
  48 .BR sched_setaffinity (2),
  49 the kernel might change the CPU at any time.
  50 (Normally this does not happen
  51 because the scheduler tries to minimize movements between CPUs to
  52 keep caches hot, but it is possible.)
  53 The caller must allow for the possibility that the information returned in
  54 .I cpu
  55 and
  56 .I node
  57 is no longer current by the time the call returns.
  58 .SH RETURN VALUE
  59 On success, 0 is returned.
  60 On error, \-1 is returned, and
  61 .I errno
  62 is set appropriately.
  63 .SH ERRORS
  64 .TP
  65 .B EFAULT
  66 Arguments point outside the calling process's address space.
  67 .SH VERSIONS
  68 .BR getcpu ()
  69 was added in kernel 2.6.19 for x86-64 and i386.
  70 Library support was added in glibc 2.29
  71 (Earlier glibc versions did not provide a wrapper for this system call,
  72 necessitating the use of
  73 .BR syscall (2).)
  74 .SH CONFORMING TO
  75 .BR getcpu ()
  76 is Linux-specific.
  77 .SH NOTES
  78 Linux makes a best effort to make this call as fast as possible.
  79 (On some architectures, this is done via an implementation in the
  80 .BR vdso (7).)
  81 The intention of
  82 .BR getcpu ()
  83 is to allow programs to make optimizations with per-CPU data
  84 or for NUMA optimization.
  85 .PP
  86 The
  87 .I tcache
  88 argument is unused since Linux 2.6.24.
  89 .\" commit 4307d1e5ada595c87f9a4d16db16ba5edb70dcb1
  90 .\" Author: Ingo Molnar <mingo@elte.hu>
  91 .\" Date:   Wed Nov 7 18:37:48 2007 +0100
  92 .\" x86: ignore the sys_getcpu() tcache parameter
  93 In earlier kernels,
  94 if this argument was non-NULL,
  95 then it specified a pointer to a caller-allocated buffer in thread-local
  96 storage that was used to provide a caching mechanism for
  97 .BR getcpu ().
  98 Use of the cache could speed
  99 .BR getcpu ()
 100 calls, at the cost that there was a very small chance that
 101 the returned information would be out of date.
 102 The caching mechanism was considered to cause problems when
 103 migrating threads between CPUs, and so the argument is now ignored.
 104 .\"
 105 .\" ===== Before kernel 2.6.24: =====
 106 .\" .I tcache
 107 .\" is a pointer to a
 108 .\" .IR "struct getcpu_cache"
 109 .\" that is used as a cache by
 110 .\" .BR getcpu ().
 111 .\" The caller should put the cache into a thread-local variable
 112 .\" if the process is multithreaded,
 113 .\" because the cache cannot be shared between different threads.
 114 .\" .I tcache
 115 .\" can be NULL.
 116 .\" If it is not NULL
 117 .\" .BR getcpu ()
 118 .\" will use it to speed up operation.
 119 .\" The information inside the cache is private to the system call
 120 .\" and should not be accessed by the user program.
 121 .\" The information placed in the cache can change between kernel releases.
 122 .\"
 123 .\" When no cache is specified
 124 .\" .BR getcpu ()
 125 .\" will be slower,
 126 .\" but always retrieve the current CPU and node information.
 127 .\" With a cache
 128 .\" .BR getcpu ()
 129 .\" is faster.
 130 .\" However, the cached information is updated only once per jiffy (see
 131 .\" .BR time (7)).
 132 .\" This means that the information could theoretically be out of date,
 133 .\" although in practice the scheduler's attempt to maintain
 134 .\" soft CPU affinity means that the information is unlikely to change
 135 .\" over the course of the caching interval.
 136 .SH SEE ALSO
 137 .BR mbind (2),
 138 .BR sched_setaffinity (2),
 139 .BR set_mempolicy (2),
 140 .BR sched_getcpu (3),
 141 .BR cpuset (7),
 142 .BR vdso (7)