[tip: sched/core] sched/cache: Avoid publishing partial LLC utilization

[tip-bot2 for Jemmy Wong]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     fcceb7da94659695d6cba97190e18e87d9cce82c
Gitweb:        https://git.kernel.org/tip/fcceb7da94659695d6cba97190e18e87d9cce82c
Author:        Jemmy Wong <jemmywong512@xxxxxxxxx>
AuthorDate:    Tue, 13 Oct 2026 16:43:38 +08:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 16 Oct 2026 09:49:04 +02:00

sched/cache: Avoid publishing partial LLC utilization

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.

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.

Skip publication on affinity retries using LBF_ALL_PINNED, retaining
the first-pass snapshots and avoiding repeated cpumask checks. On the
first pass, publish either shared LLC statistic only when env->cpus
covers the whole group. 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.

Fixes: f025ef275388 ("sched/cache: Record per LLC utilization to guide cache aware scheduling decisions")
Suggested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Suggested-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Signed-off-by: Jemmy Wong <jemmywong512@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://patch.msgid.link/20261013084338.55100-1-jemmywong512@xxxxxxxxx
---
 kernel/sched/fair.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7bf3193..a90e48d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12084,6 +12084,14 @@ static void record_sg_llc_stats(struct lb_env *env,
 		return;
 
 	/*
+	 * A redo repeats stats published on the first pass. Hotplug can
+	 * still leave env->cpus partial, so util would not match capacity.
+	 */
+	if ((env->flags & LBF_ALL_PINNED) ||
+	    !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
 	 * shared LLC domain.