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

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Message-ID: <20160524102928.GF27946@e105550-lin.cambridge.arm.com>
Date:	Tue, 24 May 2016 11:29:29 +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 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 the overall idea is that symmetric capacity systems, everything
should work as normal. For asymmetric capacity systems, we restrict
select_idle_sibling() to only look among same-capacity cpus and then use
wake_cap() to use find_idlest_{group, cpu}() to look wider if we think
should look for cpu with higher capacity than the previous one. So, for
assymmetric cpus we take one of the two routes depending on whether a
cpu of the same capacity as the previous one is okay.

Do that make any sense?

> 
> >
> > cc: Ingo Molnar <mingo@...hat.com>
> > cc: Peter Zijlstra <peterz@...radead.org>
> >
> > Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
> > ---
> >  kernel/sched/core.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index d9619a3..558ec4a 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -6410,6 +6410,9 @@ sd_init(struct sched_domain_topology_level *tl, int cpu)
> >                 sd->idle_idx = 1;
> >         }
> >
> > +       if (sd->flags & SD_ASYM_CPUCAPACITY)
> > +               sd->flags &= ~SD_WAKE_AFFINE;
> > +
> >         sd->private = &tl->data;
> >
> >         return sd;
> > --
> > 1.9.1
> >