Subject: Re: [PATCH 06/11] powerpc/smp: Generalize 2nd sched domain

From: Gautham R Shenoy <[email-masked]>
Date: Mon, 20 Jul 2020 14:37:02 +0530

On Mon, Jul 20, 2020 at 11:49:11AM +0530, Srikar Dronamraju wrote:
> * Gautham R Shenoy <[email-masked]> [2020-07-17 12:07:55]:
> 
> > On Tue, Jul 14, 2020 at 10:06:19AM +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.
> > > 
> > > 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]>
> > > ---
> > > @@ -867,11 +869,16 @@ static const struct cpumask *smallcore_smt_mask(int cpu)
> > >  }
> > >  #endif
> > > 
> > > +static const struct cpumask *cpu_bigcore_mask(int cpu)
> > > +{
> > > +	return cpu_core_mask(cpu);
> > 
> > It should be cpu_smt_mask() if we want the redundant big-core to be
> > degenerated in favour of the SMT level on P8, no? Because
> > cpu_core_mask refers to all the CPUs that are in the same chip.
> > 
> 
> Right, but it cant be cpu_smt_mask since cpu_smt_mask is only enabled in
> CONFIG_SCHED_SMT. I was looking at using sibling_map, but we have to careful
> for power9 / PowerNV mode. Guess that should be fine.

Ok.

> 
> > > +}
> > > +
> > >  static struct sched_domain_topology_level powerpc_topology[] = {
> > >  #ifdef CONFIG_SCHED_SMT
> > >  	{ cpu_smt_mask, powerpc_smt_flags, SD_INIT_NAME(SMT) },
> > >  #endif
> > > -	{ shared_cache_mask, powerpc_shared_cache_flags, SD_INIT_NAME(CACHE) },
> > > +	{ cpu_bigcore_mask, SD_INIT_NAME(BIGCORE) },
> > >  	{ cpu_cpu_mask, SD_INIT_NAME(DIE) },
> > >  	{ NULL, },
> > >  };
> > > @@ -1319,7 +1326,6 @@ static void add_cpu_to_masks(int cpu)
> > >  void start_secondary(void *unused)
> > >  {
> > >  	unsigned int cpu = smp_processor_id();
> > > -	struct cpumask *(*sibling_mask)(int) = cpu_sibling_mask;
> > > 
> > >  	mmgrab(&init_mm);
> > >  	current->active_mm = &init_mm;
> > > @@ -1345,14 +1351,20 @@ 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;
> > 
> > Shouldn't we use cpumask_subset() here ?
> 
> Wouldn't cpumask_subset should return 1 if both are same?

When are caches shared ? When the sibling_mask(cpu)  is a
strict-subset of cpu_l2_cache_mask(cpu). cpumask_weight() only
checks if the number of CPUs in cpu_l2_cache_mask(cpu) is greater than
sibling_mask(cpu) but not if constituent CPUs of the former forms
a strict superset of the latter.

We are better off using
if (!cpumask_equal(sibling_mask(cpu), mask) &&
    cpumask_subset(sibling_mask(cpu), mask))

which is accurate.

> We dont want to have shared_caches set if both the masks are equal.

> 
> >   			
> > > +	}
> > > 
> > >  	set_numa_node(numa_cpu_lookup_table[cpu]);
> > >  	set_numa_mem(local_memory_node(numa_cpu_lookup_table[cpu]));
> > > @@ -1390,6 +1402,14 @@ void __init smp_cpus_done(unsigned int max_cpus)
> > >  		smp_ops->bringup_done();
> > > 
> > >  	dump_numa_cpu_topology();
> > > +	if (shared_caches) {
> > > +		pr_info("Using shared cache scheduler topology\n");
> > > +		powerpc_topology[bigcore_idx].mask = shared_cache_mask;
> > > +#ifdef CONFIG_SCHED_DEBUG
> > > +		powerpc_topology[bigcore_idx].name = "CACHE";
> > > +#endif
> > > +		powerpc_topology[bigcore_idx].sd_flags = powerpc_shared_cache_flags;
> > > +	}
> > 
> > 
> > I would much rather that we have all the topology-fixups done in one
> > function.
> > 
> > fixup_topology(void) {
> >      if (has_big_core)
> >         powerpc_topology[smt_idx].mask = smallcore_smt_mask;
> > 
> >     if (shared_caches) {
> >        const char *name = "CACHE";
> >        powerpc_topology[bigcore_idx].mask = shared_cache_mask;
> >        strlcpy(powerpc_topology[bigcore_idx].name, name,
> >        				strlen(name));
> >        powerpc_topology[bigcore_idx].sd_flags = powerpc_shared_cache_flags;
> >     }
> > 
> >     /* Any other changes to the topology structure here */
> 
> We could do this.
> 
> > 
> > And also as an optimization, get rid of degenerate structures here
> > itself so that we don't pay additional penalty while building the
> > sched-domains each time.
> > 
> 
> Yes this is definitely in plan, but slightly later in time.
>

Ah, ok. Fine in that case.

> Thanks for the review and comments.
> 
> -- 
> Thanks and Regards
> Srikar Dronamraju