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 Srikar,

On 5/26/2026 10:28 AM, Srikar Dronamraju wrote:
> L2 Cache reported here is for SMT8 Core aka CACHE domain.

Apart for the scheduler, nothing in tree currently cares about
cpu_coregroup_mask() except for drivers/base/arch_topology.c but
Power doesn't select GENERIC_ARCH_TOPOLOGY.

Why can't Power have an internal mask for MC domain (tl_mc_mask) and
the scheduler can use cpu_coregroup_mask() for the actual LLc? (The L2
mask in this case.)

Power anyways adds its own topology via set_sched_topology() so the
default_topology from kernel/sched/topology.c remains unused.

...

> Shouldnt cache-aware scheduling be worried about cpuset partitions too.
> If a cpuset has subset of LLC cores, then we should Scheduler assume it can
> control complete LLC?

Well, the scheduling takes care of partitions and the cache aware
scheduling bits take care of looking at the full system perspective
for stats aggregation and pointing to a particular LLc.

We don't compare llc_id across cpusets so we keeping one unique llc_id
per H/W LLC instance is feasible and it enables us to keep llc_id space
limited for optimizing cache-aware scheduling.

Now if we have threads of same process across partitions, we'll
still aggregate the utilization numbers across the full LLC but
the load balancers at individual partitions will make a call on
the aggregation.

-- 
Thanks and Regards,
Prateek