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SubjectRe: [RFC PATCH v3 09/10] sched/fair: Select an energy-efficient CPU on task wake-up
On 08/06/18 12:19, Quentin Perret wrote:
> On Friday 08 Jun 2018 at 12:24:46 (+0200), Juri Lelli wrote:
> > Hi,
> >
> > On 21/05/18 15:25, Quentin Perret wrote:
> >
> > [...]
> >
> > > +static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> > > +{
> > > + unsigned long cur_energy, prev_energy, best_energy, cpu_cap, task_util;
> > > + int cpu, best_energy_cpu = prev_cpu;
> > > + struct sched_energy_fd *sfd;
> > > + struct sched_domain *sd;
> > > +
> > > + sync_entity_load_avg(&p->se);
> > > +
> > > + task_util = task_util_est(p);
> > > + if (!task_util)
> > > + return prev_cpu;
> > > +
> > > + /*
> > > + * Energy-aware wake-up happens on the lowest sched_domain starting
> > > + * from sd_ea spanning over this_cpu and prev_cpu.
> > > + */
> > > + sd = rcu_dereference(*this_cpu_ptr(&sd_ea));
> > > + while (sd && !cpumask_test_cpu(prev_cpu, sched_domain_span(sd)))
> > > + sd = sd->parent;
> > > + if (!sd)
> > > + return -1;
> >
> > Shouldn't this be return prev_cpu?
>
> Well, you shouldn't be entering this function without an sd_ea pointer,
> so this case is a sort of bug I think. By returning -1 I think we should
> end-up picking a CPU using select_fallback_rq(), which sort of makes
> sense ?

I fear cpumask_test_cpu() and such won't be happy with a -1 arg.
If it's a recoverable bug, I'd say return prev and WARN_ON_ONCE() ?

> > > +
> > > + if (cpumask_test_cpu(prev_cpu, &p->cpus_allowed))
> > > + prev_energy = best_energy = compute_energy(p, prev_cpu);
> > > + else
> > > + prev_energy = best_energy = ULONG_MAX;
> > > +
> > > + for_each_freq_domain(sfd) {
> > > + unsigned long spare_cap, max_spare_cap = 0;
> > > + int max_spare_cap_cpu = -1;
> > > + unsigned long util;
> > > +
> > > + /* Find the CPU with the max spare cap in the freq. dom. */
> >
> > I undestand this being a heuristic to cut some overhead, but shouldn't
> > the model tell between packing vs. spreading?
>
> Ah, that's a very interesting one :-) !
>
> So, with only active costs of the CPUs in the model, we can't really
> tell what's best between packing or spreading between identical CPUs if
> the migration of the task doesn't change the OPP request.
>
> In a frequency domain, all the "best" CPU candidates for a task are
> those for which we'll request a low OPP. When there are several CPUs for
> which the OPP request will be the same, we just don't know which one to
> pick from an energy standpoint, because we don't have other energy costs
> (for idle states for ex) to break the tie.
>
> With this EM, the interesting thing is that if you assume that OPP
> requests follow utilization, you are _guaranteed_ that the CPU with
> the max spare capacity in a freq domain will always be among the best
> candidates of this freq domain. And since we don't know how to
> differentiate those candidates, why not using this one ?
>
> Yes, it _might_ be better from an energy standpoint to pack small tasks
> on a CPU in order to let other CPUs go in deeper idle states. But that
> also hurts your chances to go cluster idle. Which solution is the best ?
> It depends, and we have no ways to tell with this EM.
>
> This approach basically favors cluster-packing, and spreading inside a
> cluster. That should at least be a good thing for latency, and this is
> consistent with the idea that most of the energy savings come from the
> asymmetry of the system, and not so much from breaking the tie between
> identical CPUs. That's also the reason why EAS is enabled only if your
> system has SD_ASYM_CPUCAPACITY set, as we already discussed for patch
> 05/10 :-).
>
> Does that make sense ?

Yes, thanks for the explanation. It would probably make sense to copy
and paste your text above somewhere in comment/doc for future ref.

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