Re: [process scheduler] Possible bug in context_swich()?

From: Mike Galbraith
Date: Wed Sep 08 2010 - 22:32:26 EST


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@xxxxxxxxxx>
> 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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