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

Gautham R Shenoy
2020-07-22T06:21:35Z

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@lists.ozlabs.org>
> Cc: LKML <linux-kernel@vger.kernel.org>
> Cc: Michael Ellerman <michaele@au1.ibm.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Valentin Schneider <valentin.schneider@arm.com>
> Cc: Nick Piggin <npiggin@au1.ibm.com>
> Cc: Oliver OHalloran <oliveroh@au1.ibm.com>
> Cc: Nathan Lynch <nathanl@linux.ibm.com>
> Cc: Michael Neuling <mikey@linux.ibm.com>
> Cc: Anton Blanchard <anton@au1.ibm.com>
> Cc: Gautham R Shenoy <ego@linux.vnet.ibm.com>
> Cc: Vaidyanathan Srinivasan <svaidy@linux.ibm.com>
> Cc: Jordan Niethe <jniethe5@gmail.com>
> Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> ---
> 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.