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

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Date:	Tue, 10 May 2016 07:26:23 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Mike Galbraith <mgalbraith@...e.de>
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, May 09, 2016 at 11:39:05AM +0200, Mike Galbraith wrote:
> 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.

By cpu reservation, you mean the various averages in select_task_rq_fair?
It does seem a lot of cleanup should be done.
 
> > > > 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.

I have to think this through, hot-potato. Maybe even droping it does not
sound outrageous.