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

From: Srikar Dronamraju <[email-masked]>
Date: Wed, 22 Jul 2020 12:27:47 +0530

* Gautham R Shenoy <[email-masked]> [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.

> > 
> >  	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.

> --
> Thanks and Regards
> gautham.

-- 
Thanks and Regards
Srikar Dronamraju