linux-kernel - Re: [process scheduler] Possible bug in context

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Date:	Thu, 9 Sep 2010 13:25:19 +0200
From:	fabio de francesco <fabio@...anix.org>
To:	Tim Blechmann <tim@...ngt.org>, LKML <linux-kernel@...r.kernel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [process scheduler] Possible bug in context_swich()?

On Thursday 09 September 2010 13:12:18 Tim Blechmann wrote:
> On Thursday, September 09, 2010 12:39:06 pm Peter Zijlstra wrote:
> > On Thu, 2010-09-09 at 04:32 +0200, Mike Galbraith wrote:
> > > On Wed, 2010-09-08 at 17:54 +0200, Peter Zijlstra wrote:
> > > > On Wed, 2010-09-08 at 17:28 +0200, fabio de francesco wrote:
> > > > > In context_switch() (in linux/kernel/sched.c), starting with
> > > > > release 2.6.33, two "unlikely" macro  have been changed to
> > > > > "likely". I think the previous logic was right while the latter is
> > > > > wrong.
> > > > > 
> > > > > In case I am missing something I, please, ask someone to explain
> > > > > the above mentioned inversion of logic through releases.
> > > > 
> > > > It helps if you CC people, LKML alone is a bit of a gamble.
> > > > 
> > > > git blame kernel/sched.c, will tell you that the change you refer to
> > > > comes from:
> > > > 
> > > > commit 710390d90f143a9ebb87a475215140f426792efd
> > > > Author: Tim Blechmann <tim@...ngt.org>
> > > > Date:   Tue Nov 24 11:55:27 2009 +0100
> > > > 
> > > >     sched: Optimize branch hint in context_switch()
> > > >     
> > > >     Branch hint profiling on my nehalem machine showed over 90%
> > > 
> > > >     incorrect branch hints:
> > > That change never made any sense to me, seems Tim must have been
> > > measuring a kthread load.  I benched at the time, and saw absolutely
> > > zero difference one way or the other wrt max ctx rate on my Q6600.
> > 
> > One option is to simply remove the whole branch hint.. But lets ask Tim
> > what kind of workload he used..
> 
> i was using a standard desktop workload, nothing special ...

There could have been just kernel threads ready to run... Could a server or 
workstation workload behave differently?

fabio

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