Subject: Re: [PATCH v5 06/10] powerpc/smp: Optimize start_secondary

From: Gautham R Shenoy <[email-masked]>
Date: Wed, 12 Aug 2020 11:07:49 +0530

Hi Srikar,

On Mon, Aug 10, 2020 at 12:48:30PM +0530, Srikar Dronamraju wrote:
> In start_secondary, even if shared_cache was already set, system does a
> redundant match for cpumask. This redundant check can be removed by
> checking if shared_cache is already set.
> 
> While here, localize the sibling_mask variable to within the if
> condition.
> 
> Cc: linuxppc-dev <[email-masked]>
> Cc: LKML <[email-masked]>
> Cc: Michael Ellerman <[email-masked]>
> Cc: Nicholas Piggin <[email-masked]>
> Cc: Anton Blanchard <[email-masked]>
> Cc: Oliver O'Halloran <[email-masked]>
> Cc: Nathan Lynch <[email-masked]>
> Cc: Michael Neuling <[email-masked]>
> Cc: Gautham R Shenoy <[email-masked]>
> Cc: Ingo Molnar <[email-masked]>
> Cc: Peter Zijlstra <[email-masked]>
> Cc: Valentin Schneider <[email-masked]>
> Cc: Jordan Niethe <[email-masked]>
> Cc: Vaidyanathan Srinivasan <[email-masked]>
> Signed-off-by: Srikar Dronamraju <[email-masked]>

The change looks good to me.

Reviewed-by: Gautham R. Shenoy <[email-masked]>

> ---
> Changelog v4 ->v5:
> 	Retain cache domain, no need for generalization
> 		 (Michael Ellerman, Peter Zijlstra,
> 		 Valentin Schneider, Gautham R. Shenoy)
> 
> Changelog v1 -> v2:
> 	Moved shared_cache topology fixup to fixup_topology (Gautham)
> 
>  arch/powerpc/kernel/smp.c | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 0c960ce3be42..91cf5d05e7ec 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -851,7 +851,7 @@ static int powerpc_shared_cache_flags(void)
>   */
>  static const struct cpumask *shared_cache_mask(int cpu)
>  {
> -	return cpu_l2_cache_mask(cpu);
> +	return per_cpu(cpu_l2_cache_map, cpu);
>  }
> 
>  #ifdef CONFIG_SCHED_SMT
> @@ -1305,7 +1305,6 @@ 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;
> @@ -1331,14 +1330,20 @@ 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;
> +	}
> 
>  	set_numa_node(numa_cpu_lookup_table[cpu]);
>  	set_numa_mem(local_memory_node(numa_cpu_lookup_table[cpu]));
> -- 
> 2.18.2
>