linux-kernel - Re: sched: tweak select_idle_sibling to look for idle threads

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Date:	Mon, 11 Apr 2016 06:54:21 +0200
From:	Mike Galbraith <mgalbraith@...e.de>
To:	Chris Mason <clm@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: sched: tweak select_idle_sibling to look for idle threads

On Sun, 2016-04-10 at 15:55 -0400, Chris Mason wrote:
> On Sun, Apr 10, 2016 at 12:04:21PM +0200, Mike Galbraith wrote:
> > On Sat, 2016-04-09 at 15:05 -0400, Chris Mason wrote:
> > 
> > > This does preserve the existing logic to prefer idle cores over idle
> > > CPU threads, and includes some tests to try and avoid the idle scan when we're
> > > actually better off sharing a non-idle CPU with someone else.
> > 
> > My box says the "oh nevermind" checks aren't selective enough, tbench
> > dropped 4% at clients=cores, and 2% at clients=threads.
> 
> Ok, I was able to reproduce this by stuffing tbench_srv and tbench onto
> just socket 0.  Version 2 below fixes things for me, but I'm hoping
> someone can suggest a way to get task_hot() buddy checks without the rq
> lock.
> 
> I haven't run this on production loads yet, but our 4.0 patch for this
> uses task_hot(), so I'd expect it to be on par.  If this doesn't fix it
> for you, I'll dig up a similar machine on Monday.

My box stopped caring.  I personally would be reluctant to apply it
without a "you asked for it" button or a large pile of benchmark
results.  Lock banging or not, full scan existing makes me nervous.

	-Mike