Subject: Re: [PATCH v4 06/10] powerpc/smp: Generalize 2nd sched domain

From: Michael Ellerman <[email-masked]>
Date: Fri, 31 Jul 2020 17:45:37 +1000

Srikar Dronamraju <[email-masked]> writes:
> 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.

PeterZ asked for some overview of what you're doing and why, you
responded to his mail, but I was expecting to see that text incorporated
here somewhere.

He also asked for some comments, which I would also like to see.

I'm also not clear why we want to rename it to "bigcore", that's not a
commonly understood term, I don't think it's clear to new readers what
it means.

Leaving it as the shared cache domain, and having a comment mentioning
that "bigcores" share a cache, would be clearer I think.

cheers

> 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]>
> Signed-off-by: Srikar Dronamraju <[email-masked]>
> ---
> Changelog v1 -> v2:
> 	Moved shared_cache topology fixup to fixup_topology (Gautham)
>
>  arch/powerpc/kernel/smp.c | 48 +++++++++++++++++++++++++++------------
>  1 file changed, 34 insertions(+), 14 deletions(-)
>
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index d997c7411664..3c5ccf6d2b1c 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,
> +};
> +
>  #define MAX_THREAD_LIST_SIZE	8
>  #define THREAD_GROUP_SHARE_L1   1
>  struct thread_groups {
> @@ -851,13 +859,7 @@ static int powerpc_shared_cache_flags(void)
>   */
>  static const struct cpumask *shared_cache_mask(int cpu)
>  {
> -	if (shared_caches)
> -		return cpu_l2_cache_mask(cpu);
> -
> -	if (has_big_cores)
> -		return cpu_smallcore_mask(cpu);
> -
> -	return per_cpu(cpu_sibling_map, cpu);
> +	return per_cpu(cpu_l2_cache_map, cpu);
>  }
>  
>  #ifdef CONFIG_SCHED_SMT
> @@ -867,11 +869,16 @@ static const struct cpumask *smallcore_smt_mask(int cpu)
>  }
>  #endif
>  
> +static const struct cpumask *cpu_bigcore_mask(int cpu)
> +{
> +	return per_cpu(cpu_sibling_map, cpu);
> +}
> +
>  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, },
>  };
> @@ -1311,7 +1318,6 @@ 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;
> @@ -1337,14 +1343,20 @@ 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]));
> @@ -1375,9 +1387,17 @@ static void fixup_topology(void)
>  #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
> +	if (shared_caches) {
> +		pr_info("Using shared cache scheduler topology\n");
> +		powerpc_topology[bigcore_idx].mask = shared_cache_mask;
> +		powerpc_topology[bigcore_idx].sd_flags = powerpc_shared_cache_flags;
> +#ifdef CONFIG_SCHED_DEBUG
> +		powerpc_topology[bigcore_idx].name = "CACHE";
> +#endif
> +	}
>  }
>  
>  void __init smp_cpus_done(unsigned int max_cpus)
> -- 
> 2.17.1