linux-kernel - Re: [PATCH v4 1/2] sched/fair: Check a task has a fitting cpu when updating misfit

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Date: Fri, 26 Jan 2024 15:15:39 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Qais Yousef <qyousef@...alina.io>
Cc: Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, linux-kernel@...r.kernel.org, 
	Pierre Gondois <Pierre.Gondois@....com>
Subject: Re: [PATCH v4 1/2] sched/fair: Check a task has a fitting cpu when
 updating misfit

On Fri, 26 Jan 2024 at 03:07, Qais Yousef <qyousef@...alina.io> wrote:
>
> On 01/25/24 18:50, Vincent Guittot wrote:
> > On Wed, 24 Jan 2024 at 23:38, Qais Yousef <qyousef@...alina.io> wrote:
> > >
> > > On 01/23/24 18:22, Vincent Guittot wrote:
> > >
> > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > > index bcea3d55d95d..0830ceb7ca07 100644
> > > > > --- a/kernel/sched/fair.c
> > > > > +++ b/kernel/sched/fair.c
> > > > > @@ -5065,17 +5065,61 @@ static inline int task_fits_cpu(struct task_struct *p, int cpu)
> > > > >
> > > > >  static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
> > > > >  {
> > > > > +       unsigned long uclamp_min, uclamp_max;
> > > > > +       unsigned long util, cpu_cap;
> > > > > +       int cpu = cpu_of(rq);
> > > > > +
> > > > >         if (!sched_asym_cpucap_active())
> > > > >                 return;
> > > > >
> > > > > -       if (!p || p->nr_cpus_allowed == 1) {
> > > > > -               rq->misfit_task_load = 0;
> > > > > -               return;
> > > > > -       }
> > > > > +       if (!p || p->nr_cpus_allowed == 1)
> > > > > +               goto out;
> > > > >
> > > > > -       if (task_fits_cpu(p, cpu_of(rq))) {
> > > > > -               rq->misfit_task_load = 0;
> > > > > -               return;
> > > > > +       cpu_cap = arch_scale_cpu_capacity(cpu);
> > > > > +
> > > > > +       /* If we can't fit the biggest CPU, that's the best we can ever get. */
> > > > > +       if (cpu_cap == SCHED_CAPACITY_SCALE)
> > > > > +               goto out;
> > > > > +
> > > > > +       uclamp_min = uclamp_eff_value(p, UCLAMP_MIN);
> > > > > +       uclamp_max = uclamp_eff_value(p, UCLAMP_MAX);
> > > > > +       util = task_util_est(p);
> > > > > +
> > > > > +       if (util_fits_cpu(util, uclamp_min, uclamp_max, cpu) > 0)
> > > > > +               goto out;
> > > > > +
> > > > > +       /*
> > > > > +        * If the task affinity is not set to default, make sure it is not
> > > > > +        * restricted to a subset where no CPU can ever fit it. Triggering
> > > > > +        * misfit in this case is pointless as it has no where better to move
> > > > > +        * to. And it can lead to balance_interval to grow too high as we'll
> > > > > +        * continuously fail to move it anywhere.
> > > > > +        */
> > > > > +       if (!cpumask_equal(p->cpus_ptr, cpu_possible_mask)) {
> > > > > +               unsigned long clamped_util = clamp(util, uclamp_min, uclamp_max);
> > > > > +               bool has_fitting_cpu = false;
> > > > > +               struct asym_cap_data *entry;
> > > > > +
> > > > > +               rcu_read_lock();
> > > > > +               list_for_each_entry_rcu(entry, &asym_cap_list, link) {
> > > >
> > > > Do we really want to potentially do this loop at every pick_next task ?
> > >
> > > The common case should return quickly as the biggest CPU should be present
> > > in every task by default. And after sorting the biggest CPU will be the first
> > > entry and we should return after one check.
> > >
> > > Could we move the update to another less expensive location instead?
> >
> > TBH, I don't know. I would need time to think about this...
> > May be when we set the new affinity of the task
>
> I was thinking to actually call update_misfit_status() from another less
> expensive location.
>
> We can certainly do something to help the check less expensive if we must do it
> in pick_next_task(). For example set a flag if the task belongs to a single
> capacity value; and store the highest capacity its affinity belongs too. But
> with cpuset v1, v2 and hotplug I am wary that might get messy.

I think it worth looking at such solution as this would mean parsing
the possible max capacity for the task only once per affinity change

>
> >
> > >
> > > We could try to do better tracking for CPUs that has their affinity changed,
> > > but I am not keen on sprinkling more complexity else where to deal with this.
> > >
> > > We could keep the status quouo and just prevent the misfit load balancing from
> > > increment nr_failed similar to newidle_balance too. I think this should have
> >
> > One main advantage is that we put the complexity out of the fast path
>
> How about when we update_load_avg()? After all it's the util the decides if we
> become misfit. So it makes sense to do the check when we update the util for
> the task.
>
> Which reminds me of another bug. We need to call update_misfit_status() when
> uclamp values change too.
>
> >
> > > a similar effect. Not ideal but if this is considered too expensive still
> > > I can't think of other options that don't look ugly to me FWIW.
> > >
> > >
> > > Thanks
> > >
> > > --
> > > Qais Yousef