Re: [PATCH] sched/cache: Honor asym packing over cache aware scheduling on hybrid system

[Kayra Cizmeci <kayracizmeci@gmail.com>]

Hi Tim :>,

> > Let's say when entering can_migrate_llc_task(), dst_cpu is CPU0, while src_cpu is CPU1.
> > And CPU0 has a bigger asym_prio than CPU1. No SMT. When entering can_migrate_llc_task() and sched_asym()
> > from there sched_use_asym_prio() returns true without checking if the core is fully idle or not.

> sched_asym() does check whether the destination core is idle in sched_use_asym_prio() for non SMT domain.
> 
>                   (false for non-SMT sd)           (check idle core)
>        return sd->flags & SD_SHARE_CPUCAPACITY || is_core_idle(cpu);
> 
> That is also a pre-condition for setting group_asym_packing.

Sorry for not showing the code earlier, here it is: 

static inline bool is_core_idle(int cpu)
{
	int sibling;

	for_each_cpu(sibling, cpu_smt_mask(cpu)) {
		if (cpu == sibling)
			continue;

		if (!idle_cpu(sibling))
			return false;
	}

	return true;
}
static bool sched_use_asym_prio(struct sched_domain *sd, int cpu)
{
	if (!(sd->flags & SD_ASYM_PACKING))
		return false;

	if (!sched_smt_active())
		return true;

	return sd->flags & SD_SHARE_CPUCAPACITY || is_core_idle(cpu);
}

On sched_use_asym_prio(), before the idle check a CPU without SMT returns true.
But I'm actually wrong on that one because on that one we're laying
on the idle protection outside to the CPU. If there are not any
other brothers, and I'm idle then my brother-family
is idle... Ah I messed up describing this.

But is there are any idle protection outside? I couldn't
find any. I could be missing something tho.

> > And sched_asym_prefer() comes back true too, so sched_asym() returns true and, we just returned mig_unrestricted.
> >
> > I could be missing something, If I'm not tho is that on purpose? If it is, the last paragraph needs to change
> > since it says that "asym packing tries to migrate task to an empty core" and after that "higher performing idle core"

> I think I did try to point out the idle core aspect in my commit log:

> "When asym packing tries to migrate task to an empty
> core that has higher priority than source cpu, let asym packing win.
> Moving tasks to a higher performing idle core will buy more
> performance than cache co-location."

I know. I was trying to say that if we're not choosing an idle
core this needs to change.

Thanks,
Kayra