linux-kernel - Re: [PATCH v2] sched/fair: fix 1 task per CPU

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Message-ID: <CAKfTPtAzDXKatZN_Wu-J3W8YX9bWcRxeg5CSXSxV-eRjq4vRAQ@mail.gmail.com>
Date:   Fri, 14 Sep 2018 18:26:59 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Morten Rasmussen <Morten.Rasmussen@....com>
Subject: Re: [PATCH v2] sched/fair: fix 1 task per CPU

On Fri, 14 Sep 2018 at 05:22, Valentin Schneider
<valentin.schneider@....com> wrote:
>
> Hi,
>
> On 10/09/18 07:43, Vincent Guittot wrote:
> > When CPUs have different capacity because of RT/DL tasks or
> > micro-architecture or max frequency differences, there are situation where
> > the imbalance is not correctly set to migrate waiting task on the idle CPU.
> >
> > The UC uses the force_balance case :
> >       if (env->idle != CPU_NOT_IDLE && group_has_capacity(env, local) &&
> >           busiest->group_no_capacity)
> >               goto force_balance;
> >
> > But calculate_imbalance fails to set the right amount of load to migrate
> > a task because of the special condition:
> >   busiest->avg_load <= sds->avg_load || local->avg_load >= sds->avg_load)
> >
> > Add in fix_small_imbalance, this special case that triggered the force
> > balance in order to make sure that the amount of load to migrate will be
> > enough.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
>
> Other than the commit nit, LGTM. Out of curiosity I ran some kernel compile
> on my HiKey960 (-j8) but didn't see much change - something along the lines
> of ~1% speedup, and although it was consistent over a few iterations, I'd
> need a whole lot more of them to back this up.
>
> I kind of expected it because some sporadic task can show up and tip the
> scale in the right direction, so even without the patch the situation can
> "fix itself" eventually, and it becomes less noticeable on really long
> workloads.

I have seen a better stdev and shorter duration for the tests that you
used for misfit patch.
The test have been done with asym packing and the few fixes that I
sent in another patchset for asym packing

>
> I do see a difference by looking at the trace of a simple 8 100% tasks rt-app
> workload though, as I no longer see that idling LITTLE I sometimes get
> without the patch, which is what we expect, so:
>
> Tested-by: Valentin Schneider <valentin.schneider@....com>

Thanks

>
> > ---
>
> Again, I'd argue for a slightly more explicit header. As you pointed out in
> v1, it's not just long running tasks, so maybe just "fix 1 *running* task per
> CPU"? Otherwise I feel it's a tad obscure.

To be honest i don't mind about header but I don't see the benefit of
adding *running*.
So I let Peter or Ingo decide what they prefer

>
> >  kernel/sched/fair.c | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 309c93f..72bc5e8 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8048,6 +8048,20 @@ void fix_small_imbalance(struct lb_env *env, struct sd_lb_stats *sds)
> >       local = &sds->local_stat;
> >       busiest = &sds->busiest_stat;
> >
> > +     /*
> > +      * There is available capacity in local group and busiest group is
> > +      * overloaded but calculate_imbalance can't compute the amount of load
> > +      * to migrate because load_avg became meaningless due to asymetric
> > +      * capacity between groups.
>
> Could you add something along the lines of "(see similar condition in
> find_busiest_group())"?
>
> In such case, we only want to migrate at
> > +      * least one tasks of the busiest group and rely of the average load
> > +      * per task to ensure the migration.
> > +      */
> > +     if (env->idle != CPU_NOT_IDLE && group_has_capacity(env, local) &&
> > +         busiest->group_no_capacity) {
> > +             env->imbalance = busiest->load_per_task;
> > +             return;
> > +     }
> > +
> >       if (!local->sum_nr_running)
> >               local->load_per_task = cpu_avg_load_per_task(env->dst_cpu);
> >       else if (busiest->load_per_task > local->load_per_task)
> >