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

[Vincent Guittot]

Hi Peter, Ingo,

On Fri, 14 Sep 2018 at 18:26, Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
>
> On Fri, 14 Sep 2018 at 05:22, Valentin Schneider
> <valentin.schneider@xxxxxxx> 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@xxxxxxxxxx>
> >
> > 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@xxxxxxx>
>
> 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

This patch has been in the list for a while.
What should i do to move forward with it ?

Thanks
Vincent

>
> >
> > >  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)
> > >