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

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Date:	Wed, 1 Jun 2016 08:01:05 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, bsegall@...gle.com, pjt@...gle.com,
	morten.rasmussen@....com, vincent.guittot@...aro.org,
	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 07:07:13AM +0200, Mike Galbraith wrote:
> On Tue, 2016-05-31 at 09:31 +0800, Yuyang Du wrote:
> > On Tue, May 31, 2016 at 11:21:46AM +0200, Peter Zijlstra wrote:
> > > On Tue, May 31, 2016 at 09:11:37AM +0800, Yuyang Du wrote:
> > > > The SD_BALANCE_WAKE is irrelevant in the contexts of these two removals,
> > > > and in addition SD_BALANCE_WAKE is not and should not be set in any
> > > > sched_domain flags so far.
> > > 
> > > This Changelog doesn't make any sense...
> > 
> > How? SD_BALANCE_WAKE is not in any sched_domain flags (sd->flags), even if
> > it is, it is not used anywhere, no?
> 
> If the user chooses to set SD_BALANCE_WAKE in sd->flags, it is in fact
> used.  It's just not turned on by default due to full balance on every
> wakeup being far too painful to do by default.

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.