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

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Message-ID: <1324712566.5025.12.camel@marge.simson.net>
Date:	Sat, 24 Dec 2011 08:42:46 +0100
From:	Mike Galbraith <efault@....de>
To:	Артем Анисимов 
	<aanisimov@...ox.ru>
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 Sat, 2011-12-24 at 10:26 +0300, Артем Анисимов wrote:

>   I believe that the described behaviour is erroneous and in order to
> > lessen the odds of it [waker] being preempted again soon
> one should not mess with utime but rather use another technique. It is also 
> interesting why the *user time* needs to be affected? It is inside the kernel 
> that the tracer process is woken up, so when
> > handing a few wakeup expense cycles to the wakee
> why not account those cycles as stime?

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

>   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.

	-Mike

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