Re: [PATCH 05/11] powerpc/smp: Dont assume l2-cache to be superset of sibling

["Oliver O'Halloran" <oohall@gmail.com>]

On Tue, Jul 14, 2020 at 2:45 PM Srikar Dronamraju
<srikar@linux.vnet.ibm.com> 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.

It's been a while since I looked, but I'm pretty sure the scheduler
requires child domains to be subsets of their parents. Why is this
necessary or a good idea?

> Cc: linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
> Cc: Michael Ellerman <michaele@au1.ibm.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>
> Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> ---
>  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 7d430fc536cc..875f57e41355 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -1198,6 +1198,7 @@ static bool update_mask_by_l2(int cpu, struct cpumask *(*mask_fn)(int))
>         struct device_node *l2_cache, *np;
>         int i;
>
> +       cpumask_set_cpu(cpu, mask_fn(cpu));

?

>         l2_cache = cpu_to_l2cache(cpu);
>         if (!l2_cache)
>                 return false;
> @@ -1284,29 +1285,30 @@ 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) {
> +               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;
> +       }
>
>         for_each_cpu(i, cpu_online_mask)
>                 if (get_physical_package_id(i) == pkg_id)
> --
> 2.17.1
>