linux-kernel - Re: [PATCH 06/16] sched: Disable WAKE_AFFINE for asymmetric configurations

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Message-ID: <20160524133642.GH27946@e105550-lin.cambridge.arm.com>
Date:	Tue, 24 May 2016 14:36:43 +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 06/16] sched: Disable WAKE_AFFINE for asymmetric
 configurations

On Tue, May 24, 2016 at 03:27:05PM +0200, Vincent Guittot wrote:
> On 24 May 2016 at 15:16, Morten Rasmussen <morten.rasmussen@....com> wrote:
> > On Tue, May 24, 2016 at 02:12:38PM +0200, Vincent Guittot wrote:
> >> On 24 May 2016 at 12:29, Morten Rasmussen <morten.rasmussen@....com> wrote:
> >> > On Tue, May 24, 2016 at 11:10:28AM +0200, Vincent Guittot wrote:
> >> >> On 23 May 2016 at 12:58, Morten Rasmussen <morten.rasmussen@....com> wrote:
> >> >> > If the system has cpu of different compute capacities (e.g. big.LITTLE)
> >> >> > let affine wakeups be constrained to cpus of the same type.
> >> >>
> >> >> Can you explain why you don't want wake affine with cpus with
> >> >> different compute capacity ?
> >> >
> >> > I should have made the overall idea a bit more clear. The idea is to
> >> > deal with cross-capacity migrations in the find_idlest_{group, cpu}{}
> >> > path so we don't have to touch select_idle_sibling().
> >> > select_idle_sibling() is critical for wake-up latency, and I'm assumed
> >> > that people wouldn't like adding extra overhead in there to deal with
> >> > capacity and utilization.
> >>
> >> So this means that we will never use the quick path of
> >> select_idle_sibling for cross capacity migration but always the one
> >> with extra overhead?
> >
> > Yes. select_idle_sibling() is only used to choose among equal capacity
> > cpus (capacity_orig).
> >
> >> Patch 9 adds more tests for enabling wake_affine path. Can't it also
> >> be used for cross capacity migration ? so we can use wake_affine if
> >> the task or the cpus (even with different capacity) doesn't need this
> >> extra overhead
> >
> > The test in patch 9 is to determine whether we are happy with the
> > capacity of the previous cpu, or we should go look for one with more
> > capacity. I don't see how we can use select_idle_sibling() unmodified
> > for sched domains containing cpus of different capacity to select an
> > appropriate cpu. It is just picking an idle cpu, it might have high
> > capacity or low, it wouldn't care.
> >
> > How would you avoid the overhead of checking capacity and utilization of
> > the cpus and still pick an appropriate cpu?
> 
> My point is that there is some wake up case where we don't care about
> the capacity and utilization of cpus even for cross capacity migration
> and we will never take benefit of this fast path.
> You have added an extra check for setting want_affine in patch 9 which
> uses capacity and utilization of cpu to disable this fast path when a
> task needs more capacity than available. Can't you use this function
> to disable the want_affine for cross-capacity migration situation that
> cares of the capacity and need the full scan of sched_domain but keep
> it enable for other cases ?

It is not clear to me what the other cases are. What kind of cases do
you have in mind?