[thirdparty/man-pages.git] / man2 / getcpu.2

.\" This man page is Copyright (C) 2006 Andi Kleen <ak@muc.de>.
.\"
.\" %%%LICENSE_START(VERBATIM_ONE_PARA)
.\" Permission is granted to distribute possibly modified copies
.\" of this page provided the header is included verbatim,
.\" and in case of nontrivial modification author and date
.\" of the modification is added to the header.
.\" %%%LICENSE_END
.\"
.\" 2008, mtk, various edits
.\"
.TH GETCPU 2 2019-03-06 "Linux" "Linux Programmer's Manual"
.SH NAME
getcpu \- determine CPU and NUMA node on which the calling thread is running
.SH SYNOPSIS
.nf
.B #include <linux/getcpu.h>
.PP
.BI "int getcpu(unsigned *" cpu ", unsigned *" node \
", struct getcpu_cache *" tcache );
.fi
.SH DESCRIPTION
The
.BR 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
.I cpu
and
.I 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
.I cpu
or
.I node
is NULL nothing is written to the respective pointer.
.PP
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).
.PP
The information placed in
.I cpu
is guaranteed to be current only at the time of the call:
unless the CPU affinity has been fixed using
.BR 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
.I cpu
and
.I node
is no longer current by the time the call returns.
.SH RETURN VALUE
On success, 0 is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EFAULT
Arguments point outside the calling process's address space.
.SH VERSIONS
.BR 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
.BR syscall (2).)
.SH CONFORMING TO
.BR getcpu ()
is Linux-specific.
.SH 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
.BR vdso (7).)
The intention of
.BR getcpu ()
is to allow programs to make optimizations with per-CPU data
or for NUMA optimization.
.PP
The
.I tcache
argument is unused since Linux 2.6.24.
.\" commit 4307d1e5ada595c87f9a4d16db16ba5edb70dcb1
.\" Author: Ingo Molnar <mingo@elte.hu>
.\" Date:   Wed Nov 7 18:37:48 2007 +0100
.\" x86: ignore the sys_getcpu() tcache parameter
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
.BR getcpu ().
Use of the cache could speed
.BR 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.
.\"
.\" ===== Before kernel 2.6.24: =====
.\" .I tcache
.\" is a pointer to a
.\" .IR "struct getcpu_cache"
.\" that is used as a cache by
.\" .BR getcpu ().
.\" The caller should put the cache into a thread-local variable
.\" if the process is multithreaded,
.\" because the cache cannot be shared between different threads.
.\" .I tcache
.\" can be NULL.
.\" If it is not NULL
.\" .BR getcpu ()
.\" will use it to speed up operation.
.\" The information inside the cache is private to the system call
.\" and should not be accessed by the user program.
.\" The information placed in the cache can change between kernel releases.
.\"
.\" When no cache is specified
.\" .BR getcpu ()
.\" will be slower,
.\" but always retrieve the current CPU and node information.
.\" With a cache
.\" .BR getcpu ()
.\" is faster.
.\" However, the cached information is updated only once per jiffy (see
.\" .BR time (7)).
.\" This means that the information could theoretically be out of date,
.\" although in practice the scheduler's attempt to maintain
.\" soft CPU affinity means that the information is unlikely to change
.\" over the course of the caching interval.
.SH SEE ALSO
.BR mbind (2),
.BR sched_setaffinity (2),
.BR set_mempolicy (2),
.BR sched_getcpu (3),
.BR cpuset (7),
.BR vdso (7)
Commit	Line	Data
	1	.\" This man page is Copyright (C) 2006 Andi Kleen <ak@muc.de>.
	2	.\"
	3	.\" %%%LICENSE_START(VERBATIM_ONE_PARA)
	4	.\" Permission is granted to distribute possibly modified copies
	5	.\" of this page provided the header is included verbatim,
	6	.\" and in case of nontrivial modification author and date
	7	.\" of the modification is added to the header.
	8	.\" %%%LICENSE_END
	9	.\"
	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)