sched/fair: Attach sched_domain_shared to sd_asym_cpucapacity

On asymmetric CPU capacity systems, the wakeup path uses
select_idle_capacity(), which scans the span of sd_asym_cpucapacity
rather than sd_llc.

The has_idle_cores hint however lives on sd_llc->shared, so the
wakeup-time read of has_idle_cores operates on an LLC-scoped blob while
the actual scan/decision spans the asym domain; nr_busy_cpus also lives
in the same shared sched_domain data, but it's never used in the asym
CPU capacity scenario.

Therefore, move the sched_domain_shared object to sd_asym_cpucapacity
whenever the CPU has a SD_ASYM_CPUCAPACITY_FULL ancestor and that
ancestor is non-overlapping (i.e., not built from SD_NUMA). In that case
the scope of has_idle_cores matches the scope of the wakeup scan.

Fall back to attaching the shared object to sd_llc in three cases:

  1) plain symmetric systems (no SD_ASYM_CPUCAPACITY_FULL anywhere);

  2) CPUs in an exclusive cpuset that carves out a symmetric capacity
     island: has_asym is system-wide but those CPUs have no
     SD_ASYM_CPUCAPACITY_FULL ancestor in their hierarchy and follow
     the symmetric LLC path in select_idle_sibling();

  3) exotic topologies where SD_ASYM_CPUCAPACITY_FULL lands on an
     SD_NUMA-built domain. init_sched_domain_shared() keys the shared
     blob off cpumask_first(span), which on overlapping NUMA domains
     would alias unrelated spans onto the same blob. Keep the shared
     object on the LLC there; select_idle_capacity() gracefully skips
     the has_idle_cores preference when sd->shared is NULL.

While at it, also rename the per-CPU sd_llc_shared to sd_balance_shared,
as it is no longer strictly tied to the LLC.

Co-developed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Acked-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://patch.msgid.link/20260516055850.1345932-1-arighi@nvidia.com

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