linux-kernel - Re: [PATCH v2 07/13] sched/fair: Let asymmetric cpu configurations balance at wake-up

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Message-ID: <20160715083732.GE21816@e105550-lin.cambridge.arm.com>
Date:	Fri, 15 Jul 2016 09:37:33 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Yuyang Du <yuyang.du@...el.com>, mgalbraith@...e.de,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 07/13] sched/fair: Let asymmetric cpu configurations
 balance at wake-up

On Thu, Jul 14, 2016 at 03:45:17PM +0200, Vincent Guittot wrote:
> On 13 July 2016 at 18:14, Morten Rasmussen <morten.rasmussen@....com> wrote:
> > On Wed, Jul 13, 2016 at 02:56:41PM +0200, Vincent Guittot wrote:
> >> On 22 June 2016 at 19:03, Morten Rasmussen <morten.rasmussen@....com> wrote:
> >> > Currently, SD_WAKE_AFFINE always takes priority over wakeup balancing if
> >> > SD_BALANCE_WAKE is set on the sched_domains. For asymmetric
> >> > configurations SD_WAKE_AFFINE is only desirable if the waking task's
> >> > compute demand (utilization) is suitable for all the cpu capacities
> >> > available within the SD_WAKE_AFFINE sched_domain. If not, let wakeup
> >>
> >> instead of "suitable for all the cpu capacities available within the
> >> SD_WAKE_AFFINE sched_domain", should it be "suitable for local cpu and
> >> prev cpu" becasue you only check the capacity of these 2 CPUs.
> >
> > Good point. I currently make the implicit assumption that capacity of local cpu
> > and prev cpu represent the capacity for all cpus their SD_WAKE_AFFINE
> > domains. It breaks if you should choose to have SD_WAKE_AFFINE on a
> > domain that spans both little and big cpus, as if local/prev cpu happens
> > to be big we assume that they are all big and let select_idle_sibling()
> > handle the task placement even for big tasks if local/prev cpu are both
> > big.
> 
> Isn't the sd_llc used in select_idle_sibling and not the
> SD_WAKE_AFFINE domian so if CPUs in the sd_llc are homogeneous, we are
> safe

Yes, I confused myself (again) with SD_WAKE_AFFINE and sd_llc in the
above. It should have been sd_llc instead of SD_WAKE_AFFINE. I will fix
the commit message to be correct.