Re: [PATCH v2 06/10] powerpc/smp: Generalize 2nd sched domain

Srikar Dronamraju
2020-07-22T07:40:03Z

* Gautham R Shenoy <ego@linux.vnet.ibm.com> [2020-07-22 12:26:40]:

> Hello Srikar,
>
> On Tue, Jul 21, 2020 at 05:08:10PM +0530, Srikar Dronamraju wrote:
> > Currently "CACHE" domain happens to be the 2nd sched domain as per
> > powerpc_topology. This domain will collapse if cpumask of l2-cache is
> > same as SMT domain. However we could generalize this domain such that it
> > could mean either be a "CACHE" domain or a "BIGCORE" domain.
> >
> > While setting up the "CACHE" domain, check if shared_cache is already
> > set.
> > @@ -1339,14 +1345,20 @@ void start_secondary(void *unused)
> > /* Update topology CPU masks */
> > add_cpu_to_masks(cpu);
> >
> > - if (has_big_cores)
> > - sibling_mask = cpu_smallcore_mask;
> > /*
> > * Check for any shared caches. Note that this must be done on a
> > * per-core basis because one core in the pair might be disabled.
> > */
> > - if (!cpumask_equal(cpu_l2_cache_mask(cpu), sibling_mask(cpu)))
> > - shared_caches = true;
> > + if (!shared_caches) {
> > + struct cpumask *(*sibling_mask)(int) = cpu_sibling_mask;
> > + struct cpumask *mask = cpu_l2_cache_mask(cpu);
> > +
> > + if (has_big_cores)
> > + sibling_mask = cpu_smallcore_mask;
> > +
> > + if (cpumask_weight(mask) > cpumask_weight(sibling_mask(cpu)))
> > + shared_caches = true;
>
> At the risk of repeating my comment to the v1 version of the patch, we
> have shared caches only l2_cache_mask(cpu) is a strict superset of
> sibling_mask(cpu).
>
> "cpumask_weight(mask) > cpumask_weight(sibling_mask(cpu))" does not
> capture this.

Why would it not? We are setting shared_caches if and only if l2_cache_mask
is a strict superset of sibling/smallcore mask.

> Could we please use
>
> if (!cpumask_equal(sibling_mask(cpu), mask) &&
> cpumask_subset(sibling_mask(cpu), mask) {
> }
>

Scheduler mandates that two cpumasks for the same CPU would either have to
be equal or one of them has to be a strict superset of the other. If not the
scheduler would mark our domains as broken. That being the case,
cpumask_weight will correctly capture what we are looking for. That said
your condition is also correct but slightly heavier and doesn't provide us
with any more information or correctness.

>
> Otherwise the patch looks good to me.
>
> --
> Thanks and Regards
> gautham.

--
Thanks and Regards
Srikar Dronamraju