linux-kernel - Re: [PATCH 07/16] sched: Make SD_BALANCE

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Message-ID: <20160525092727.GK27946@e105550-lin.cambridge.arm.com>
Date:	Wed, 25 May 2016 10:27:27 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	peterz@...radead.org, mingo@...hat.com, dietmar.eggemann@....com,
	vincent.guittot@...aro.org, mgalbraith@...e.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/16] sched: Make SD_BALANCE_WAKE a topology flag

On Wed, May 25, 2016 at 07:52:49AM +0800, Yuyang Du wrote:
> On Mon, May 23, 2016 at 11:58:49AM +0100, Morten Rasmussen wrote:
> > For systems with the SD_ASYM_CPUCAPACITY flag set on higher level in the
> > sched_domain hierarchy we need a way to enable wake-up balancing for the
> > lower levels as well as we may want to balance tasks that don't fit the
> > capacity of the previous cpu.
> > 
> > We have the option of introducing a new topology flag to express this
> > requirement, or let the existing SD_BALANCE_WAKE flag be set by the
> > architecture as a topology flag. The former means introducing yet
> > another flag, the latter breaks the current meaning of topology flags.
> > None of the options are really desirable.
>  
> I'd propose to replace SD_WAKE_AFFINE with SD_BALANCE_WAKE. And the
> SD_WAKE_AFFINE semantic is simply "waker allowed":
> 
> waker_allowed = cpumask_test_cpu(cpu, tsk_cpus_allowed(p));
> 
> This can be implemented without current functionality change.
> 
> From there, the choice between waker and wakee, and fast path
> select_idle_sibling() and the rest slow path should be reworked, which
> I am thinking about.

I don't really understand how that would work. If you change the
semantics of the flags you don't preserve current behaviour. To me it
sounds like at total rewrite of everything.

SD_BALANCE_WAKE controls whether we go slow path or not in case
want_affine is false. SD_WAKE_AFFINE controls whether we should consider
waking up near the waker instead of always waking up near the previous
cpu.