Re: [PATCH v2 05/10] powerpc/smp: Dont assume l2-cache to be superset of sibling

Gautham R Shenoy
2020-07-24T07:10:57Z

On Wed, Jul 22, 2020 at 12:27:47PM +0530, Srikar Dronamraju wrote:
> * Gautham R Shenoy <ego@linux.vnet.ibm.com> [2020-07-22 11:51:14]:
>
> > Hi Srikar,
> >
> > > diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> > > index 72f16dc0cb26..57468877499a 100644
> > > --- a/arch/powerpc/kernel/smp.c
> > > +++ b/arch/powerpc/kernel/smp.c
> > > @@ -1196,6 +1196,7 @@ static bool update_mask_by_l2(int cpu, struct cpumask *(*mask_fn)(int))
> > > if (!l2_cache)
> > > return false;
> > >
> > > + cpumask_set_cpu(cpu, mask_fn(cpu));
> >
> >
> > Ok, we need to do this because "cpu" is not yet set in the
> > cpu_online_mask. Prior to your patch the "cpu" was getting set in
> > cpu_l2_cache_map(cpu) as a side-effect of the code that is removed in
> > the patch.
> >
>
> Right.
>
> >
> > > for_each_cpu(i, cpu_online_mask) {
> > > /*
> > > * when updating the marks the current CPU has not been marked
> > > @@ -1278,29 +1279,30 @@ static void add_cpu_to_masks(int cpu)
> > > * add it to it's own thread sibling mask.
> > > */
> > > cpumask_set_cpu(cpu, cpu_sibling_mask(cpu));
> > > + cpumask_set_cpu(cpu, cpu_core_mask(cpu));
>
> Note: Above, we are explicitly setting the cpu_core_mask.

You are right. I missed this.

>
> > >
> > > for (i = first_thread; i < first_thread + threads_per_core; i++)
> > > if (cpu_online(i))
> > > set_cpus_related(i, cpu, cpu_sibling_mask);
> > >
> > > add_cpu_to_smallcore_masks(cpu);
> > > - /*
> > > - * Copy the thread sibling mask into the cache sibling mask
> > > - * and mark any CPUs that share an L2 with this CPU.
> > > - */
> > > - for_each_cpu(i, cpu_sibling_mask(cpu))
> > > - set_cpus_related(cpu, i, cpu_l2_cache_mask);
> > > update_mask_by_l2(cpu, cpu_l2_cache_mask);
> > >
> > > - /*
> > > - * Copy the cache sibling mask into core sibling mask and mark
> > > - * any CPUs on the same chip as this CPU.
> > > - */
> > > - for_each_cpu(i, cpu_l2_cache_mask(cpu))
> > > - set_cpus_related(cpu, i, cpu_core_mask);
> > > + if (pkg_id == -1) {
> >
> > I suppose this "if" condition is an optimization, since if pkg_id != -1,
> > we anyway set these CPUs in the cpu_core_mask below.
> >
> > However...
>
> This is not just an optimization.
> The hunk removed would only work if cpu_l2_cache_mask is bigger than
> cpu_sibling_mask. (this was the previous assumption that we want to break)
> If the cpu_sibling_mask is bigger than cpu_l2_cache_mask and pkg_id is -1,
> then setting only cpu_l2_cache_mask in cpu_core_mask will result in a broken
> topology.
>
> >
> > > + struct cpumask *(*mask)(int) = cpu_sibling_mask;
> > > +
> > > + /*
> > > + * Copy the sibling mask into core sibling mask and
> > > + * mark any CPUs on the same chip as this CPU.
> > > + */
> > > + if (shared_caches)
> > > + mask = cpu_l2_cache_mask;
> > > +
> > > + for_each_cpu(i, mask(cpu))
> > > + set_cpus_related(cpu, i, cpu_core_mask);
> > >
> > > - if (pkg_id == -1)
> > > return;
> > > + }
> >
> >
> > ... since "cpu" is not yet set in the cpu_online_mask, do we not miss setting
> > "cpu" in the cpu_core_mask(cpu) in the for-loop below ?
> >
> >
>
> As noted above, we are setting before. So we don't missing the cpu and hence
> have not different from before.

Fair enough.

>
> > --
> > Thanks and Regards
> > gautham.
>
> --
> Thanks and Regards
> Srikar Dronamraju