[PATCH] sched/cache: Avoid publishing partial LLC utilization

[Jemmy Wong]

update_sg_lb_stats() sums utilization over the CPUs in both
sched_group_span(group) and env->cpus, but takes capacity from
group->sgc->capacity for the whole group. record_sg_llc_stats()
publishes both.

Rejecting an all-affinity-pinned source runqueue removes that CPU from
env->cpus and retries group selection. The retry can publish the
partial utilization with the full capacity. On a two-CPU LLC this
replaces 1800/2048 with 900/2048.

can_migrate_llc() then treats a busy preferred LLC as having spare
capacity. More tasks can aggregate into it, migrations out can be
blocked, and two busy LLCs may not fall back to generic balancing.
This increases CPU contention and task latency. Found by inspection.

Publish either shared LLC statistic only when env->cpus covers the
whole group. A partial scan keeps the previous complete snapshot;
other groups still refresh. Load balancing still uses the
affinity-filtered statistics.

The same window exists during CPU hotplug. env->cpus drops a CPU at
set_cpu_active(false), but the group span and sgc->capacity still
include it until cpuset_cpu_inactive() rebuilds the domains. Keeping
the previous snapshot avoids the mismatched publish. After the rebuild,
the next periodic balance records both statistics again.

Signed-off-by: Jemmy Wong <jemmywong512@xxxxxxxxx>
---
 kernel/sched/fair.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index aa4a7d22d1df..a98a62f729d4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12121,6 +12121,13 @@ static void record_sg_llc_stats(struct lb_env *env,
 	if (env->sd->child != rcu_dereference_all(per_cpu(sd_llc, env->dst_cpu)))
 		return;

+	/*
+	 * Affinity retries can remove CPUs from env->cpus. Do not publish
+	 * partial utilization against the capacity of the whole group.
+	 */
+	if (!cpumask_subset(sched_group_span(group), env->cpus))
+		return;
+
 	/*
 	 * At this point we know this group spans a LLC domain.
 	 * Record the statistic of this group in its corresponding
--
2.25.1