Re: [BUG] sched/cache: "Make LLC id continuous" causes NULL cpumask dereference in build_sched_domains on POWER9

[K Prateek Nayak <kprateek.nayak@amd.com>]

Hello Chenyu, Venkat,

On 5/25/2026 9:05 PM, Chen, Yu C wrote:
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 3467f86fd78f..cf6c2e4190ab 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -1042,11 +1042,6 @@ static const struct cpumask *tl_smallcore_smt_mask(struct sched_domain_topology_
>  }
>  #endif
> 
> -struct cpumask *cpu_coregroup_mask(int cpu)
> -{
> -       return per_cpu(cpu_coregroup_map, cpu);
> -}
> -
>  static bool has_coregroup_support(void)
>  {
>         /* Coregroup identification not available on shared systems */
> @@ -1056,6 +1051,14 @@ static bool has_coregroup_support(void)
>         return coregroup_enabled;
>  }
> 
> +struct cpumask *cpu_coregroup_mask(int cpu)
> +{
> +       if (!has_coregroup_support())
> +               return cpu_l2_cache_mask(cpu);
> +> +       return per_cpu(cpu_coregroup_map, cpu);

Interestingly, on powerpc, the MC domain doesn't have SD_SHARE_LLC flag
set but I believe there is still some benefit of keeping the tasks on
the same hemisphere?

If we are actually aiming for LLC, I think cpu_l2_cache_mask() is the
right cpumask for all cases since tl_cache_mask() also returns that
and the l2_cache_mask is set in all cases covered by update_mask_by_l2()
in the same file.

If consolidation on hemisphere is beneficial, then the above diff
looks correct.

Note: For has_coregroup_support(), with the above fix, the scheduler
side llc_id will now be based on MC domain's span which seems wrong
since it doesn't have SD_SHARE_LLC flag and might lead to other
behavioral changes now.

> +}
> +
>  static int __init init_big_cores(void)
>  {
>         int cpu;

-- 
Thanks and Regards,
Prateek