linux-kernel - Re: [git pull] scheduler fixes

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Message-ID: <4972E860.5080206@redhat.com>
Date:	Sun, 18 Jan 2009 10:29:20 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Mike Galbraith <efault@....de>,
	Kevin Shanahan <kmshanah@...b.org.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	a.p.zijlstra@...llo.nl, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [git pull] scheduler fixes

Ingo Molnar wrote:
> * Mike Galbraith <efault@....de> wrote:
>
>   
>> On Sat, 2009-01-17 at 04:43 -0800, Andrew Morton wrote:
>>     
>>> http://bugzilla.kernel.org/show_bug.cgi?id=12465 just popped up - another
>>> scheduler regression.  It has been bisected.
>>>       
>> Seems pretty clear.  I'd suggest reverting it.
>>     
>
> We can revert it (and will revert it if no solution is found), but i'd 
> also like to understand why it happens, because that kind of regression 
> from this change is unexpected - we might be hiding some bug that could 
> pop up under less debuggable circumstances, so we need to understand it 
> while we have a chance.
>
> Below is the commit in question. Avi, any ideas what makes KVM special 
> here? Perhaps its use of "preempt notifiers" is causing a problem somehow?
>   

preempt notifiers use should cause additional context switch costs of a 
few thousand cycles and possible an IPI (if a vcpu was migrated).  So 
I'd suspect scheduling latency here.

Is it possible to trace this (the time between a wake up and actual 
scheduling of a task)?

-- 
error compiling committee.c: too many arguments to function

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