linux-kernel - Re: [PATCH 03/16] sched/fair: Disregard idle task wakee_flips in wake

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Date:	Wed, 1 Jun 2016 21:57:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Mike Galbraith <mgalbraith@...e.de>, mingo@...hat.com,
	dietmar.eggemann@....com, yuyang.du@...el.com,
	vincent.guittot@...aro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/16] sched/fair: Disregard idle task wakee_flips in
 wake_wide

On Mon, May 23, 2016 at 01:00:10PM +0100, Morten Rasmussen wrote:
> On Mon, May 23, 2016 at 01:12:07PM +0200, Mike Galbraith wrote:
> > On Mon, 2016-05-23 at 11:58 +0100, Morten Rasmussen wrote:
> > > wake_wide() is based on task wakee_flips of the waker and the wakee to
> > > decide whether an affine wakeup is desirable. On lightly loaded systems
> > > the waker is frequently the idle task (pid=0) which can accumulate a lot
> > > of wakee_flips in that scenario. It makes little sense to prevent affine
> > > wakeups on an idle cpu due to the idle task wakee_flips, so it makes
> > > more sense to ignore them in wake_wide().
> > 
> > You sure?  What's the difference between a task flipping enough to
> > warrant spreading the load, and an interrupt source doing the same? 
> >  I've both witnessed firsthand, and received user confirmation of this
> > very thing improving utilization.
> 
> Right, I didn't consider the interrupt source scenario, my fault.
> 
> The problem then seems to be distinguishing truly idle and busy doing
> interrupts. The issue that I observe is that wake_wide() likes pushing
> tasks around in lightly scenarios which isn't desirable for power
> management. Selecting the same cpu again may potentially let others
> reach deeper C-state.
> 
> With that in mind I will if I can do better. Suggestions are welcome :-)

Seeing how we always so select_idle_siblings() after affine_sd, the only
wake_affine movement that matters is cross-llc.

So intra-llc wakeups can avoid the movement, no?