linux-kernel - Re: Commit 1e5a74059f9 broke utime measurement of ptraced() processes

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Date:	Tue, 27 Dec 2011 11:30:36 +0300
From:	Артем Анисимов 
	<aanisimov@...ox.ru>
To:	Mike Galbraith <efault@....de>
Cc:	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl, mingo@...e.hu
Subject: Re: Commit 1e5a74059f9 broke utime measurement of ptraced() processes

On Saturday 24 December 2011 10:42:46, Mike Galbraith wrote :

> 
> >   Also I'd like to know if there is a way to get reliable utime
> >   measurements in recent kernels.
> 
> If wakeup preemption is undesirable, you could run SCHED_BATCH.

> CPU utilization shifts with wakeup preemption, so there will be visible
> effect when you change wakeup preemption in any way.

  Thanks, SCHED_BATCH does solve my problem. I'd like to modify the man page 
times (2) so that it includes a warning on utime of processes that wake each 
other. Do you find the following note correct?

----- to be added to times (2) -----
  On Linux, the user time of a process depends on pattern of wakeups of the 
process. The scheduler is permitted to cut waker's CPU usage and hand a few 
wakeup expense cycles to the wakee that preempted. As a consequence, if two 
processes A and B wake each other frequently (e.g., they communicate via 
pipes, or by sending signals) then only the collective user time of A and B is 
well defined; individual user times of A and B can be arbitrarily 
redistributed between them. If this behaviour is undesirable then both 
processes need to use the SCHED_BATCH scheduler class, see 
sched_setscheduler(2).
----- to be added to times (2) -----
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