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

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Date:	Mon, 09 May 2016 11:39:05 +0200
From:	Mike Galbraith <mgalbraith@...e.de>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Chris Mason <clm@...com>,
	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 Mon, 2016-05-09 at 09:13 +0800, Yuyang Du wrote:
> On Mon, May 09, 2016 at 09:44:13AM +0200, Mike Galbraith wrote:

> > In a perfect world, running only Chris' benchmark on an otherwise idle
> > box, there would never _be_ any work to steal. 
> 
> What is the perfect world like? I don't get what you mean.

In a perfect world from this benchmark's perspective, when you fork or
wake while box is underutilized, wakee/child lands on an idle CPU.  To
this benchmark, anything else is broken.
 
> > In the real world, we
> > smooth utilization, optimistically peek at this/that, and intentionally
> > throttle idle balancing (etc etc), which adds up to an imperfect world
> > for this (based on real world load) benchmark.
>  
> So, is this a shout-out: these parts should be coordinated better?

Switching to instantaneous load along with the cpu reservation hackery
made Chris's benchmark a happy camper.  Is that the answer?  Nope, just
verification of the where the problem lives.

> > > En... should we try remove recording last_wakee?
> > 
> > The more the merrier, go for it! :)
>  
> Nuh, really, this heuristic is too heuristic, :) 
> The totality of all possible cases is scary.

Well, make it better.  The author provided evidence when it was born.

	-Mike