linux-kernel - Re: [PATCH RFC/TEST] sched: make sync affine wakeups work

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Message-ID: <1399009289.5233.39.camel@marge.simpson.net>
Date:	Fri, 02 May 2014 07:41:29 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, morten.rasmussen@....com,
	mingo@...nel.org, peterz@...radead.org,
	george.mccollister@...il.com, ktkhai@...allels.com
Subject: Re: [PATCH RFC/TEST] sched: make sync affine wakeups work

On Fri, 2014-05-02 at 07:32 +0200, Mike Galbraith wrote: 
> On Fri, 2014-05-02 at 00:42 -0400, Rik van Riel wrote: 
> > Currently sync wakeups from the wake_affine code cannot work as
> > designed, because the task doing the sync wakeup from the target
> > cpu will block its wakee from selecting that cpu.
> > 
> > This is despite the fact that whether or not the wakeup is sync
> > determines whether or not we want to do an affine wakeup...
> 
> If the sync hint really did mean we ARE going to schedule RSN, waking
> local would be a good thing.  It is all too often a big fat lie.

BTW, we used to have avg_overlap and a sync less heuristic to improve
the accuracy of the sync wakeup hint, but I ripped it out because it was
dead wrong far too often.  You can try reviving it, but preemption among
other things break it.  You'll have better luck with a simple context
switch rate heuristic, but that's less than perfect too.

-Mike

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