Subject: Re: [PATCH 05/11] powerpc/smp: Dont assume l2-cache to be superset of sibling

From: Gautham R Shenoy <[email-masked]>
Date: Mon, 20 Jul 2020 14:28:12 +0530

Hi Srikar,

On Mon, Jul 20, 2020 at 12:15:04PM +0530, Srikar Dronamraju wrote:
> * Gautham R Shenoy <[email-masked]> [2020-07-17 11:30:11]:
> 
> > Hi Srikar,
> > 
> > On Tue, Jul 14, 2020 at 10:06:18AM +0530, Srikar Dronamraju wrote:
> > > Current code assumes that cpumask of cpus sharing a l2-cache mask will
> > > always be a superset of cpu_sibling_mask.
> > > 
> > > Lets stop that assumption.
> > > 
> > > Cc: linuxppc-dev <[email-masked]>
> > > Cc: Michael Ellerman <[email-masked]>
> > > Cc: Nick Piggin <[email-masked]>
> > > Cc: Oliver OHalloran <[email-masked]>
> > > Cc: Nathan Lynch <[email-masked]>
> > > Cc: Michael Neuling <[email-masked]>
> > > Cc: Anton Blanchard <[email-masked]>
> > > Cc: Gautham R Shenoy <[email-masked]>
> > > Cc: Vaidyanathan Srinivasan <[email-masked]>
> > > Signed-off-by: Srikar Dronamraju <[email-masked]>
> > > ---
> > >  arch/powerpc/kernel/smp.c | 28 +++++++++++++++-------------
> > >  1 file changed, 15 insertions(+), 13 deletions(-)
> > > 
> > > diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> > > index 7d430fc536cc..875f57e41355 100644
> > > --- a/arch/powerpc/kernel/smp.c
> > > +++ b/arch/powerpc/kernel/smp.c
> > > @@ -1198,6 +1198,7 @@ static bool update_mask_by_l2(int cpu, struct cpumask *(*mask_fn)(int))
> > >  	struct device_node *l2_cache, *np;
> > >  	int i;
> > > 
> > > +	cpumask_set_cpu(cpu, mask_fn(cpu));
> > 
> > It would be good to comment why do we need to do set the CPU in the
> > l2-mask if we don't have a l2cache domain.
> > 
> 
> Good Catch, 
> We should move this after the cpu_to_l2cache.
> 
> > >  	l2_cache = cpu_to_l2cache(cpu);
> > >  	if (!l2_cache)
> > >  		return false;
> > > @@ -1284,29 +1285,30 @@ 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));
> > > 
> > >  	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) {
> > > +		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;
> > > +
> > 
> > 
> > Now that we decoupling the containment relationship between
> > sibling_mask and l2-cache mask, should we set all the CPUs that are
> > both in cpu_sibling_mask(cpu) as well as cpu_l2_mask(cpu) in
> > cpu_core_mask ? 
> > 
> 
> Are you saying instead of setting this cpu in this cpu_core_mask, can we set
> all the cpus in the mask in cpu_core_mask?

No. What I am referring to is in the for-loop below, you are setting
the CPUs that are set in mask(cpu) in the cpu_core_mask.

Now, the above code sets
mask(cpu) == cpu_sibling_mask(cpu) in the absence of shared_caches, and 
          == cpu_l2_cache_mask(cpu) in the presence of shared_cache.

Since we have decoupled the assumption that cpu_sibling_mask(cpu) may not
be contained within cpu_l2_cache_mask(cpu), in the presence of a
shared-cache, why are we only picking the CPUs in
cpu_l2_cache_mask(cpu) to be set in cpu_core_maks(cpu) ? It should
ideally be the superset whose CPUs should be set in
cpu_core_mask(cpu). And the correct cpuset is
cpumask_or(cpu_sibling_mask(cpu), cpu_l2_cache_mask(cpu))

> Currently we dont know if any of the cpus of the mask were already set or
> not. Plus we need to anyway update cpumask of all other cpus to says they
> are related. So setting a mask instead of cpu at a time will not change
> anything for our side.
> 
> > > +		for_each_cpu(i, mask(cpu))
> > > +			set_cpus_related(cpu, i, cpu_core_mask);
> > > 
> > > -	if (pkg_id == -1)
> > >  		return;
> > > +	}
> > > 
> > >  	for_each_cpu(i, cpu_online_mask)
> > >  		if (get_physical_package_id(i) == pkg_id)
> > > -- 
> > > 2.17.1
> > > 
> > --
> > Thanks and Regards
> > gautham.
> 
> -- 
> Thanks and Regards
> Srikar Dronamraju