[PATCH 1/2] sched/fair: Skip rq_offline_fair() work for CPUs that stay active

[Qiliang Yuan <qiliang.yuan@linux.dev>]

A sched domain rebuild, triggered by CPU hotplug or a cpuset partition
change, calls rq_attach_root() for every active CPU in the affected
domains. That runs set_rq_offline() and set_rq_online() with the rq lock
held and interrupts disabled, so rq_offline_fair() is called for CPUs
that are not going down.

Commit af98d8a36a96 ("sched/fair: Fix CPU bandwidth limit bypass during
CPU hotplug") made unthrottle_offline_cfs_rqs() return early for active
CPUs, but clear_tg_offline_cfs_rqs(), added by commit f60a631ab9ed
("sched/fair: Fix tg->load when offlining a CPU"), still walks every
task group and drops the CPU's tg->load_avg contribution, which
update_tg_load_avg() then adds back. With thousands of cpu cgroups each
rebuild costs O(nr_cpus * nr_task_groups).

Move the active check into rq_offline_fair() so that both walks only run
for a CPU going down.

With 4000 cpu cgroups on a 32-CPU VM booted with rt_group_sched=0,
switching a 4-CPU cpuset to a partition root and offlining or onlining
one CPU take (median of 20 partition switches and 16 hotplug operations):

  operation         before (ms)  after (ms)  change
  partition root          13.28        7.68    -42%
  cpu offline             44.95       30.96    -31%
  cpu online              36.84       28.48    -23%

Signed-off-by: Qiliang Yuan <qiliang.yuan@linux.dev>
---
 kernel/sched/fair.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 79f6b3a4fd7f3..e245a31b0544e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7816,10 +7816,6 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
 
 	lockdep_assert_rq_held(rq);
 
-	// Do not unthrottle for an active CPU
-	if (cpumask_test_cpu(cpu_of(rq), cpu_active_mask))
-		return;
-
 	/*
 	 * The rq clock has already been updated in the
 	 * set_rq_offline(), so we should skip updating
@@ -15064,6 +15060,13 @@ static void rq_offline_fair(struct rq *rq)
 {
 	update_sysctl();
 
+	/*
+	 * A sched domain rebuild moves every active CPU to a new root domain
+	 * and lands here as well. Only a CPU going down needs the work below.
+	 */
+	if (cpu_active(cpu_of(rq)))
+		return;
+
 	/* Ensure any throttled groups are reachable by pick_next_task */
 	unthrottle_offline_cfs_rqs(rq);
 

-- 
2.43.0