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

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Date:	Thu, 09 Sep 2010 04:32:19 +0200
From:	Mike Galbraith <efault@....de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	fabio de francesco <fabio@...anix.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [process scheduler] Possible bug in context_swich()?

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.

	-Mike

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