linux-kernel - Re: [RFC PATCH 1/2] sched: Clean up SD_BALANCE

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Date:	Wed, 1 Jun 2016 09:03:11 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Benjamin Segall <bsegall@...gle.com>,
	Paul Turner <pjt@...gle.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [RFC PATCH 1/2] sched: Clean up SD_BALANCE_WAKE flags in sched
 domain build-up

On Wed, Jun 01, 2016 at 10:32:53AM +0200, Vincent Guittot wrote:
> > Yup. Up to this point, we don't have any disagreement. And I don't think we
> > have any disagreement conceptually. What the next patch really does is:
> >
> > (1) we don't remove SD_BALANCE_WAKE as an important sched_domain flag, on
> >     the contrary, we strengthen it.
> >
> > (2) the semantic of SD_BALANCE_WAKE is currently represented by SD_WAKE_AFFINE,
> >     we actually remove this representation.
> >
> > (3) regarding the semantic of SD_WAKE_AFFINE, it is really not about selecting
> >     waker CPU or about the fast path. Conceptually, it is just saying the waker
> >     CPU is a valid and important candidate if SD_BALANCE_WAKE, which is just so
> >     obvious, so I don't think it deserves to be a separate sched_domain flag.
> >
> > (4) the outcome is, if SD_BALANCE_WAKE, we definitely will/should try waker CPU,
> >     and if !SD_BALANCE_WAKE, we don't try waker CPU. So nothing functional is
> >     changed.
> 
> 
> AFAIU, there is 4 possible cases during wake up:
> - we don't want any balance at wake so we don't have SD_BALANCE_WAKE
> nor SD_WAKE_AFFINE in sched_domain->flags
> - we only want wake affine balance check so we only have
> SD_WAKE_AFFINE in sched_domain->flags
> - we want wake_affine and full load balance at wake so we have both
> SD_BALANCE_WAKE and SD_WAKE_AFFINE in sched_domain->flags
> - we want  full load balance but want to skip wake affine fast path so
> we only have SD_BALANCE_WAKE in sched_domain->flags
> 
> I'm not sure that we can still do only wake_affine or only full
> load_balance with your changes whereas these sequences are valid ones

So with the patch, we will have a little bit semantic change, SD_BALANCE_WAKE
implies SD_WAKE_AFFINE if allowed, and will favor "fast path" if possible. I don't
think we should do anything otherwise.

So I think this is a combined case better than either of the "only wake_affine"
or "only full" cases. Make sense?