Re: [PATCH 2/5] sched/fair: Attach sched_domain_shared to sd_asym_cpucapacity

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 5/9/26 11:37 PM, Andrea Righi wrote:
> From: K Prateek Nayak <kprateek.nayak@amd.com>
> 
> 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.
> 
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> 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>
> ---
>   kernel/sched/fair.c     | 19 ++++++---
>   kernel/sched/sched.h    |  2 +-
>   kernel/sched/topology.c | 95 +++++++++++++++++++++++++++++++++++------
>   3 files changed, 95 insertions(+), 21 deletions(-)
> 
nit: There is comment still in fair.c still. Please fix that as well.
"sd_llc->shared->has_idle_cores and enabled through update_idle_core() above."