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prctl.2: Document PR_SET_TIMERSLACK and PR_GET_TIMERSLACK 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2012-10-25 16:42:13 +02:00
parent 8e9412e7a5
commit 934487a086
1 changed files with 66 additions and 3 deletions
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man2/prctl.2
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@ -43,11 +43,11 @@
.\" PR_TASK_PERF_EVENTS_ENABLE
.\" 2012-09-20 Kees Cook, update PR_SET_SECCOMP for mode 2
.\" 2012-09-20 Kees Cook, document PR_SET_NO_NEW_PRIVS, PR_GET_NO_NEW_PRIVS
.\" 2012-10-25 Michael Kerrisk, Document PR_SET_TIMERSLACK and
.\" PR_GET_TIMERSLACK
.\"
.\" FIXME: Document PR_SET_TIMERSLACK and PR_GET_TIMERSLACK (new in 2.6.28)
.\" commit 6976675d94042fbd446231d1bd8b7de71a980ada
.\"
.TH PRCTL 2 2012-10-22 "Linux" "Linux Programmer's Manual"
.TH PRCTL 2 2012-10-25 "Linux" "Linux Programmer's Manual"
.SH NAME
prctl \- operations on a process
.SH SYNOPSIS
@ -362,6 +362,69 @@ the "securebits" flags of the calling thread.
See
.BR capabilities (7).
.TP
.BR PR_SET_TIMERSLACK " (since Linux 2.6.28)"
.\" See https://lwn.net/Articles/369549/
.\" commit 6976675d94042fbd446231d1bd8b7de71a980ada
Set the current timer slack for the calling thread to the nanosecond value
supplied in
.IR arg2 .
If
.I arg2
is less than or equal to zero,
.\" It seems that it's not possible to set the timer slack to zero;
.\" The minimum value is 1? Seems a little strange.
reset the current timer slack to the thread's default timer slack value.
The timer slack is used by the kernel to group timer expirations
for the calling thread that are close to one another;
as a consequence, timer expirations for the thread may be
up to the specified number of nanoseconds late (but will never expire early).
Grouping timer expirations can help reduce system power consumption
by minimizing CPU wake-ups.
The timer expirations affected by timer slack are those set by
.BR select (2),
.BR pselect (2),
.BR poll (2),
.BR ppoll (2),
.BR epoll_wait (2),
.BR epoll_pwait (2),
.BR clock_nanosleep (2),
.BR nanosleep (2),
and
.BR futex (2)
(and thus the library functions implemented via futexes, including
.\" List obtained by grepping for futex usage in glibc source
.BR pthread_cond_timedwait (3),
.BR pthread_mutex_timedlock (3),
.BR pthread_rwlock_timedrdlock (3),
.BR pthread_rwlock_timedwrlock (3),
and
.BR sem_timedwait (3)).
Timer slack is not applied to threads that are scheduled under
a realtime scheduling policy (see
.BR sched_setscheduler (2)).
Each thread has two associated timer slack values:
a "default" value, and a "current" value.
The current value is the one that governs grouping
of timer expirations.
When a new thread is created,
the two timer slack values are made the same as the current value
of the creating thread.
Thereafter, a thread can adjust its current timer slack value via
.BR PR_SET_TIMERSLACK
(the default value can't be changed).
The timer slack values of
.IR init
(PID 1), the ancestor of all processes,
is 50,000 nanoseconds (50 microseconds).
The timer slack values are preserved across
.BR execve (2).
.TP
.BR PR_GET_TIMERSLACK " (since Linux 2.6.28)"
Return the current timer slack value of the calling thread.
.TP
.BR PR_SET_TIMING " (since Linux 2.6.0-test4)"
Set whether to use (normal, traditional) statistical process timing or
accurate timestamp-based process timing, by passing