sched/cache: Refresh LLC capacity across CPU hotplug, to fix capacity underestimation bug

The scheduler scales LLC capacity by the fraction of cache-sharing CPUs
covered by a domain:

  llc_bytes = cache_size * span_weight / shared_weight

During CPU teardown, sched_cpu_deactivate() rebuilds scheduler domains
before cacheinfo_cpu_pre_down() removes the CPU from shared_cpu_map. The
new domains therefore use the old sharing weight. The later call to
sched_update_llc_bytes() looks up the departing CPU's sd_llc, which has
already been detached, and returns without correcting the surviving CPUs.

On a Ryzen 5 7535U with twelve logical CPUs sharing a 16 MiB LLC,
offlining one SMT sibling left the remaining CPUs with:

  llc_bytes = floor(16777216 * 11 / 12) = 15379114 bytes

The correct capacity is still 16777216 bytes. On systems with active
cache-aware scheduling, an underestimated capacity can cause
exceed_llc_capacity() to reject aggregation for a process whose footprint
would fit. Unchanged cpuset partitions sharing the physical cache can
also retain stale capacity when a CPU comes online in another partition.

Pass the cache-sharing mask already retained by cacheinfo to the
scheduler update. Refresh every surviving CPU using its own LLC domain
so that each partition receives the correct share. This also preserves
the correction needed as cache-sharing maps grow during boot.

Keep the existing CPU-hotplug and scheduler-domain synchronization. The
update remains on the hotplug path; no steady-state scheduling operation
or persistent allocation is added.

Fixes: 7030513a0877 ("sched/cache: Calculate the LLC size and store it in sched_domain")
Signed-off-by: Davi Chaves Azevedo <davichazbh@gmail.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Chen Yu <yu.c.chen@intel.com>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Tested-by: Chen Yu <yu.c.chen@intel.com>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: <stable@kernel.org> # v7.2.x
Link: https://patch.msgid.link/6751d93e15889e624796c74db0bfe66603d60b1b.1790035273.git.tim.c.chen@linux.intel.com

1 files changed, 2 insertions, 2 deletions

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index b5d9d7c2b8ad..f96812d71c51 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -281,9 +281,9 @@ static inline int task_node(const struct task_struct *p)
 }
 
 #ifdef CONFIG_SCHED_CACHE
-extern void sched_update_llc_bytes(unsigned int cpu);
+extern void sched_update_llc_bytes(const struct cpumask *cpus);
 #else
-static inline void sched_update_llc_bytes(unsigned int cpu) { }
+static inline void sched_update_llc_bytes(const struct cpumask *cpus) { }
 #endif
 
 #endif /* _LINUX_SCHED_TOPOLOGY_H */