linux-kernel - Re: [PATCH RFC] select_idle

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Date:	Thu, 28 Apr 2016 15:17:46 +0200
From:	Mike Galbraith <mgalbraith@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Chris Mason <clm@...com>, Ingo Molnar <mingo@...nel.org>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] select_idle_sibling experiments

On Thu, 2016-04-28 at 14:00 +0200, Peter Zijlstra wrote:
> On Wed, Apr 06, 2016 at 09:27:24AM +0200, Mike Galbraith wrote:
> > sched: ratelimit nohz
> > 
> > Entering nohz code on every micro-idle is too expensive to bear.
> > 
> > Signed-off-by: Mike Galbraith <efault@....de>
> 
> > +int sched_needs_cpu(int cpu)
> > +{
> > +> > 	> > if (tick_nohz_full_cpu(cpu))
> > +> > 	> > 	> > return 0;
> > +
> > +> > 	> > return  cpu_rq(cpu)->avg_idle < sysctl_sched_migration_cost;
> 
> So the only problem I have with this patch is the choice of limit. This
> isn't at all tied to the migration cost.

Yup.

> And some people are already twiddling with the migration_cost knob to
> affect the idle_balance() behaviour -- making it much more agressive by
> dialing it down. When you do that you also loose the effectiveness of
> this proposed usage, even though those same people would probably want
> this.
> 
> Failing a spot of inspiration for a runtime limit on this; we might have
> to introduce yet another knob :/

I'll roll one with a yet another of it's very own.

	-Mike