Re: [PATCH] sched/fair: Prefer idle CPU to cache affinity

[Vincent Guittot]

On Wed, 10 Mar 2021 at 06:53, Srikar Dronamraju
<srikar@xxxxxxxxxxxxxxxxxx> wrote:
>
> * Vincent Guittot <vincent.guittot@xxxxxxxxxx> [2021-03-08 14:52:39]:
>
> > On Fri, 26 Feb 2021 at 17:41, Srikar Dronamraju
> > <srikar@xxxxxxxxxxxxxxxxxx> wrote:
> > >
>
> Thanks Vincent for your review comments.
>
> > > +static int prefer_idler_llc(int this_cpu, int prev_cpu, int sync)
> > > +{
> > > +       struct sched_domain_shared *tsds, *psds;
> > > +       int pnr_busy, pllc_size, tnr_busy, tllc_size, diff;
> > > +
> > > +       tsds = rcu_dereference(per_cpu(sd_llc_shared, this_cpu));
> > > +       tnr_busy = atomic_read(&tsds->nr_busy_cpus);
> > > +       tllc_size = per_cpu(sd_llc_size, this_cpu);
> > > +
> > > +       psds = rcu_dereference(per_cpu(sd_llc_shared, prev_cpu));
> > > +       pnr_busy = atomic_read(&psds->nr_busy_cpus);
> > > +       pllc_size = per_cpu(sd_llc_size, prev_cpu);
> > > +
> > > +       /* No need to compare, if both LLCs are fully loaded */
> > > +       if (pnr_busy == pllc_size && tnr_busy == pllc_size)
> > > +               return nr_cpumask_bits;
> > > +
> > > +       if (sched_feat(WA_WAKER) && tnr_busy < tllc_size)
> > > +               return this_cpu;
> >
> > Why have you chosen to favor this_cpu instead of prev_cpu unlike for wake_idle ?
>
> At this point, we know the waker running on this_cpu and wakee which was
> running on prev_cpu are affine to each other and this_cpu and prev_cpu dont
> share cache. I chose to move them close to each other to benefit from the
> cache sharing. Based on feedback from Peter and Rik, I made the check more
> conservative i.e tnr_busy <= tllc_size/smt_weight (where smt_weight is the
> cpumask weight of smt domain for this_cpu) i.e if we have a free core in

yeah make sense

> this llc domain, chose this_cpu.  select_idle_sibling() should pick an idle
> cpu/core/smt within the llc domain for this_cpu.
>
> Do you feel, this may not be the correct option?

I was worried that we end up pulling tasks in same llc but the
condition above and wake_wide should prevent such behavior

>
> We are also experimenting with another option, were we call prefer_idler_cpu
> after wa_weight. I.e
> 1. if wake_affine_weight choses this_cpu but llc in prev_cpu has an idle
> smt/CPU but there are no idle smt/CPU in this_cpu, then chose idle smt/CPU
> in prev_cpu
> 2. if wake_affine_weight choses nr_cpumask(aka prev_cpu) but llc in this_cpu
> has an idle smt/CPU but there are no idle smt/CPU in prev_cpu, then chose
> idle smt/CPU in this_cpu
>
>
> > > +
> > > +       /* For better wakeup latency, prefer idler LLC to cache affinity */
> > > +       diff = tnr_busy * pllc_size - sync - pnr_busy * tllc_size;
> > > +       if (!diff)
> > > +               return nr_cpumask_bits;
> > > +       if (diff < 0)
> > > +               return this_cpu;
> > > +
> > > +       return prev_cpu;
> > > +}
> > > +
> > >  static int wake_affine(struct sched_domain *sd, struct task_struct *p,
> > >                        int this_cpu, int prev_cpu, int sync)
> > >  {
> > > @@ -5877,6 +5907,10 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p,
> > >         if (sched_feat(WA_IDLE))
> > >                 target = wake_affine_idle(this_cpu, prev_cpu, sync);
> > >
> > > +       if (sched_feat(WA_IDLER_LLC) && target == nr_cpumask_bits &&
> > > +                               !cpus_share_cache(this_cpu, prev_cpu))
> > > +               target = prefer_idler_llc(this_cpu, prev_cpu, sync);
> >
> > could you use the same naming convention as others function ?
> > wake_affine_llc as an example
>
> I guess you meant s/prefer_idler_llc/wake_affine_llc/

yes

> Sure. I can modify.
>
> >
> > > +
> > >         if (sched_feat(WA_WEIGHT) && target == nr_cpumask_bits)
> > >                 target = wake_affine_weight(sd, p, this_cpu, prev_cpu, sync);
> > >
> > > @@ -5884,8 +5918,11 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p,
> > >         if (target == nr_cpumask_bits)
> > >                 return prev_cpu;
> > >
> > > -       schedstat_inc(sd->ttwu_move_affine);
> > > -       schedstat_inc(p->se.statistics.nr_wakeups_affine);
> > > +       if (target == this_cpu) {
> >
> > How is this condition related to $subject ?
>
> Before this change, wake_affine_weight and wake_affine_idle would either
> return this_cpu or nr_cpumask_bits. Just before this check, we check if
> target is nr_cpumask_bits and return prev_cpu. So the stats were only
> incremented when target was this_cpu.
>
> However with prefer_idler_llc, we may return this_cpu, prev_cpu or
> nr_cpumask_bits. Now we only to update stats when we have chosen to migrate
> the task to this_cpu. Hence I had this check.

ok got it.

May be return earlier in this case like for  if (target ==
nr_cpumask_bits) above

>
> If we use the slightly lazier approach which is check for wa_weight first
> before wa_idler_llc, then we may not need this change at all.
>
> --
> Thanks and Regards
> Srikar Dronamraju