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

[Vincent Guittot]

On 24 May 2016 at 12:29, Morten Rasmussen <morten.rasmussen@xxxxxxx> 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@xxxxxxx> 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?
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

>
> 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@xxxxxxxxxx>
>> > cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> >
>> > Signed-off-by: Morten Rasmussen <morten.rasmussen@xxxxxxx>
>> > ---
>> >  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
>> >