RE: Re: [RFC PATCH v6 3/4] scheduler: scan idle cpu in cluster for tasks within one LLC

[Song Bao Hua (Barry Song)]

> >>
> >>> -----Original Message-----
> >>> From: Dietmar Eggemann [mailto:dietmar.eggemann@xxxxxxx]
> 
> [...]
> 
> >>> On 20/04/2021 02:18, Barry Song wrote:
> 
> [...]
> 
> >> I am really confused. The whole code has only checked if wake_flags
> >> has WF_TTWU, it has never checked if sd_domain has SD_BALANCE_WAKE flag.
> >
> > look at :
> > #define WF_TTWU     0x08 /* Wakeup;            maps to SD_BALANCE_WAKE */
> >
> > so  when wake_wide return false, we use the wake_affine mecanism but
> > if it's false then we fllback to default mode which looks for:
> > if (tmp->flags & sd_flag)
> >
> > This means looking for SD_BALANCE_WAKE which is never set
> >
> > so sd will stay NULL and you will end up calling select_idle_sibling anyway
> >
> >>
> >> static int
> >> select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
> >> {
> >>         ...
> >>
> >>         if (wake_flags & WF_TTWU) {
> >>                 record_wakee(p);
> >>
> >>                 if (sched_energy_enabled()) {
> >>                         new_cpu = find_energy_efficient_cpu(p, prev_cpu);
> >>                         if (new_cpu >= 0)
> >>                                 return new_cpu;
> >>                         new_cpu = prev_cpu;
> >>                 }
> >>
> >>                 want_affine = !wake_wide(p) && cpumask_test_cpu(cpu,
> p->cpus_ptr);
> >>         }
> >> }
> >>
> >> And try_to_wake_up() has always set WF_TTWU:
> >> static int
> >> try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> >> {
> >>         cpu = select_task_rq(p, p->wake_cpu, wake_flags | WF_TTWU);
> >>         ...
> >> }
> >>
> >> So the change in wake_wide will actually affect the value of want_affine.
> >> And I did also see code entered slow path during my benchmark.
> 
> Yes, this is happening but IMHO not for wakeups. Check wake_flags for
> the tasks which go through `slow path` on your machine. They should have
> WF_EXEC or WF_FORK, not WF_TTWU (& WF_SYNC).

Yes. Both of you are right. The slow path I reported yesterday came from
WF_FORK actually.

> 
> >> One issue I mentioned during linaro open discussion is that
> >> since I have moved to use cluster size to decide the value
> >> of wake_wide, relatively less tasks will make wake_wide()
> >> decide to go to slow path, thus, tasks begin to spread to
> >> other NUMA,  but actually llc_size might be able to contain
> >> those tasks. So a possible model might be:
> >> static int wake_wide(struct task_struct *p)
> >> {
> >>         tasksize < cluster : scan cluster
> >>         tasksize > llc      : slow path
> >>         tasksize > cluster && tasksize < llc: scan llc
> >> }
> >>
> >> thoughts?
> 
> Like Vincent explained, the return value of wake_wide() doesn't matter.
> For wakeups you always end up in sis().

Though we will never go to slow path, wake_wide() will affect want_affine,
so eventually affect the "new_cpu"?

	for_each_domain(cpu, tmp) {
		/*
		 * If both 'cpu' and 'prev_cpu' are part of this domain,
		 * cpu is a valid SD_WAKE_AFFINE target.
		 */
		if (want_affine && (tmp->flags & SD_WAKE_AFFINE) &&
		    cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) {
			if (cpu != prev_cpu)
				new_cpu = wake_affine(tmp, p, cpu, prev_cpu, sync);

			sd = NULL; /* Prefer wake_affine over balance flags */
			break;
		}

		if (tmp->flags & sd_flag)
			sd = tmp;
		else if (!want_affine)
			break;
	}

If wake_affine is false, the above won't execute, new_cpu(target) will
always be "prev_cpu"? so when "task size > cluster size" in wake_wide(),
this means we won't pull the wakee to the cluster of waker since target
is always prev_cpu? It seems sensible.

Thanks
Barry