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

From: Gautham R Shenoy <[email-masked]>
Date: Wed, 22 Jul 2020 12:26:40 +0530

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.
> 
> Cc: linuxppc-dev <[email-masked]>
> Cc: LKML <[email-masked]>
> Cc: Michael Ellerman <[email-masked]>
> Cc: Ingo Molnar <[email-masked]>
> Cc: Peter Zijlstra <[email-masked]>
> Cc: Valentin Schneider <[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]>
> Cc: Jordan Niethe <[email-masked]>
> Signed-off-by: Srikar Dronamraju <[email-masked]>
> ---
> Changelog v1 -> v2:
> powerpc/smp: Generalize 2nd sched domain
> 	Moved shared_cache topology fixup to fixup_topology (Gautham)
>

Just one comment below.

>  arch/powerpc/kernel/smp.c | 49 ++++++++++++++++++++++++++++-----------
>  1 file changed, 35 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 57468877499a..933ebdf97432 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -85,6 +85,14 @@ EXPORT_PER_CPU_SYMBOL(cpu_l2_cache_map);
>  EXPORT_PER_CPU_SYMBOL(cpu_core_map);
>  EXPORT_SYMBOL_GPL(has_big_cores);
> 
> +enum {
> +#ifdef CONFIG_SCHED_SMT
> +	smt_idx,
> +#endif
> +	bigcore_idx,
> +	die_idx,
> +};
> +

[..snip..]

> @@ -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.

Could we please use

      if (!cpumask_equal(sibling_mask(cpu), mask) &&
      	  cpumask_subset(sibling_mask(cpu), mask) {
      }

?

> +	}
> 
>  	set_numa_node(numa_cpu_lookup_table[cpu]);
>  	set_numa_mem(local_memory_node(numa_cpu_lookup_table[cpu]));
> @@ -1374,10 +1386,19 @@ int setup_profiling_timer(unsigned int multiplier)
> 
>  static void fixup_topology(void)
>  {
> +	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;
> +	}
> +
>  #ifdef CONFIG_SCHED_SMT
>  	if (has_big_cores) {
>  		pr_info("Big cores detected but using small core scheduling\n");
> -		powerpc_topology[0].mask = smallcore_smt_mask;
> +		powerpc_topology[smt_idx].mask = smallcore_smt_mask;
>  	}
>  #endif

Otherwise the patch looks good to me.

--
Thanks and Regards
gautham.