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sched.7: Document sched_rt_period_us and sched_rt_runtime_us /proc files 

And rework and relocate the text on dealing with runaway
real-time processes.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2014-05-12 11:28:23 +02:00
parent 1608b67143
commit 0c055c7506
1 changed files with 53 additions and 12 deletions
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@ -369,6 +369,59 @@ See
.BR getrlimit (2)
for further information on
.BR RLIMIT_RTPRIO .
.SS Limiting the CPU usage of real-time and deadline processes
A nonblocking infinite loop in a thread scheduled under the
.BR SCHED_FIFO ,
.BR SCHED_RR ,
or
.BR SCHED_DEADLINE
policy will block all threads with lower
priority forever.
Prior to Linux 2.6.25, the only way of preventing a runaway real-time
process from freezing the system was to run (at the console)
a shell scheduled under a higher static priority than the tested application.
This allows an emergency kill of tested
real-time applications that do not block or terminate as expected.
Since Linux 2.6.25, there are other techniques for dealing with runaway
real-time and deadline processes.
One of these is to use the
.BR RLIMIT_RTTIME
resource limit to set a ceiling on the CPU time that
a real-time process may consume.
See
.BR getrlimit (2)
for details.
Since version 2.6.25, Linux also provides two
.I /proc
files that can be used to reserve a certain amount of CPU time
to be used by non-real-time processes.
Reserving some CPU time in this fashion allows some CPU time to be
allocated to (say) a root shell that can be used to kill a runaway process.
Both of these files specify time values in microseconds:
.TP
.IR /proc/sys/kernel/sched_rt_period_us
This file specifies a scheduling period that is equivalent to
100% CPU bandwidth.
The value in this file can range from 1 to
.BR INT_MAX ,
giving an operating range of 1 microsecond to around 35 minutes.
The default value in this file is 1,000,000 (1 second).
.TP
.IR /proc/sys/kernel/sched_rt_runtime_us
The value in this file specifies how much of the "period" time
can be used by all real-time and deadline scheduled processes
on the system.
The value in this file can range from \-1 to
.BR INT_MAX \-1.
Specifying \-1 makes the runtime the same as the period;
that is, no CPU time is set aside for non-real-time processes
(which was the Linux behavior before kernel 2.6.25).
The default value in this file is 950,000 (0.95 seconds),
meaning that 5% of the CPU time is reserved for processes that
don't run under a real-time or deadline scheduling policy.
.PP
.SS Response time
A blocked high priority thread waiting for I/O has a certain
response time before it is scheduled again.
@ -388,18 +441,6 @@ paging delays; this can be done with
.BR mlock (2)
or
.BR mlockall (2).
Since a nonblocking infinite loop in a thread scheduled under
\fBSCHED_FIFO\fP or \fBSCHED_RR\fP will block all threads with lower
priority forever, a software developer should always keep available on
the console a shell scheduled under a higher static priority than the
tested application.
This will allow an emergency kill of tested
real-time applications that do not block or terminate as expected.
See also the description of the
.BR RLIMIT_RTTIME
resource limit in
.BR getrlimit (2).
.SH NOTES
.PP
Originally, Standard Linux was intended as a general-purpose operating