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

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Message-ID: <20160608084515.GD9187@e105550-lin.cambridge.arm.com>
Date:	Wed, 8 Jun 2016 09:45:16 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...hat.com, dietmar.eggemann@....com, yuyang.du@...el.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, Jun 01, 2016 at 10:18:17PM +0200, Peter Zijlstra 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.
> 
> So why can't you couple this to ASYM_CAPACITY? If that's set anywhere,
> add BALANCE_WAKE as appropriate?

That should be possible. It is set at the sched_domain levels that span
cpus of different capacity, but we need to set BALANCE_WAKE on the level
below as well. So we would have to introduce a dependency between flags
set at different levels.

However, following the discussion with Vincent on enabling WAKE_AFFINE
across cpus of different capacity, I might be able to repurpose
ASYM_CPUCAPACITY to set BALANCE_WAKE instead which would be simpler I
think.