Re: [PATCH v2 05/10] powerpc/smp: Dont assume l2-cache to be superset of sibling

[Gautham R Shenoy]

Hi Srikar,

On Tue, Jul 21, 2020 at 05:08:09PM +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. cpu_l2_cache_mask is a superset of
> cpu_sibling_mask if and only if shared_caches is set.
> 
> Cc: linuxppc-dev <linuxppc-dev@xxxxxxxxxxxxxxxx>
> Cc: LKML <linux-kernel@xxxxxxxxxxxxxxx>
> Cc: Michael Ellerman <michaele@xxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Valentin Schneider <valentin.schneider@xxxxxxx>
> Cc: Nick Piggin <npiggin@xxxxxxxxxxx>
> Cc: Oliver OHalloran <oliveroh@xxxxxxxxxxx>
> Cc: Nathan Lynch <nathanl@xxxxxxxxxxxxx>
> Cc: Michael Neuling <mikey@xxxxxxxxxxxxx>
> Cc: Anton Blanchard <anton@xxxxxxxxxxx>
> Cc: Gautham R Shenoy <ego@xxxxxxxxxxxxxxxxxx>
> Cc: Vaidyanathan Srinivasan <svaidy@xxxxxxxxxxxxx>
> Cc: Jordan Niethe <jniethe5@xxxxxxxxx>
> Signed-off-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
> ---
> Changelog v1 -> v2:
> powerpc/smp: Dont assume l2-cache to be superset of sibling
> 	Set cpumask after verifying l2-cache. (Gautham)
> 
>  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 72f16dc0cb26..57468877499a 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -1196,6 +1196,7 @@ static bool update_mask_by_l2(int cpu, struct cpumask *(*mask_fn)(int))
>  	if (!l2_cache)
>  		return false;
> 
> +	cpumask_set_cpu(cpu, mask_fn(cpu));


Ok, we need to do this because "cpu" is not yet set in the
cpu_online_mask. Prior to your patch the "cpu" was getting set in
cpu_l2_cache_map(cpu) as a side-effect of the code that is removed in
the patch.


>  	for_each_cpu(i, cpu_online_mask) {
>  		/*
>  		 * when updating the marks the current CPU has not been marked
> @@ -1278,29 +1279,30 @@ 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) {

I suppose this "if" condition is an optimization, since if pkg_id != -1,
we anyway set these CPUs in the cpu_core_mask below.

However...

> +		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;
> +
> +		for_each_cpu(i, mask(cpu))
> +			set_cpus_related(cpu, i, cpu_core_mask);
> 
> -	if (pkg_id == -1)
>  		return;
> +	}


... since "cpu" is not yet set in the cpu_online_mask, do we not miss setting
"cpu" in the cpu_core_mask(cpu) in the for-loop below ?


> 
>  	for_each_cpu(i, cpu_online_mask)
>  		if (get_physical_package_id(i) == pkg_id)


Before this patch it was unconditionally getting set in
cpu_core_mask(cpu) because of the fact that it was set in
cpu_l2_cache_mask(cpu) and we were unconditionally setting all the
CPUs in cpu_l2_cache_mask(cpu) in cpu_core_mask(cpu).

What am I missing ?

> -- 
> 2.17.1
>

--
Thanks and Regards
gautham.