[PATCH 2/2] sched/fair: Avoid re-syncing runtime_enabled on root domain changes

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

rq_online_fair() calls update_runtime_enabled(), which walks every task
group and takes each group's cfs_bandwidth lock to copy its quota state
into the CPU's cfs_rq. A sched domain rebuild reaches it for every
active CPU through rq_attach_root(), with the rq lock held and
interrupts disabled.

The walk only matters when the cached state may be stale. While a CPU
is online, tg_set_cfs_bandwidth() updates runtime_enabled on it under
the rq lock, and it holds cpus_read_lock() so it cannot miss a CPU that
is coming up. Only unthrottle_offline_cfs_rqs() clears runtime_enabled
behind its back, and a CPU that has never been online has not been
visited at all.

Track this in a per-rq flag that update_runtime_enabled() sets after
the walk and unthrottle_offline_cfs_rqs() clears, and skip the walk
while it is set.

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           7.68        1.21    -84%
  cpu offline             30.96       13.44    -57%
  cpu online              28.48       12.82    -55%

Signed-off-by: Qiliang Yuan <qiliang.yuan@linux.dev>
---
 kernel/sched/fair.c  | 11 +++++++++++
 kernel/sched/sched.h |  2 ++
 2 files changed, 13 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e245a31b0544e..e2f67bdc3031e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7798,6 +7798,15 @@ static void __maybe_unused update_runtime_enabled(struct rq *rq)
 
 	lockdep_assert_rq_held(rq);
 
+	/*
+	 * tg_set_cfs_bandwidth() keeps runtime_enabled up to date while the
+	 * CPU is online, so only the first activation and the one following
+	 * unthrottle_offline_cfs_rqs() need the walk.
+	 */
+	if (rq->cfsb_runtime_valid)
+		return;
+	rq->cfsb_runtime_valid = true;
+
 	guard(rcu)();
 
 	list_for_each_entry_rcu(tg, &task_groups, list) {
@@ -7816,6 +7825,8 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
 
 	lockdep_assert_rq_held(rq);
 
+	rq->cfsb_runtime_valid = false;
+
 	/*
 	 * The rq clock has already been updated in the
 	 * set_rq_offline(), so we should skip updating
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 1d76950dbb0a1..1a73383204384 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1412,6 +1412,8 @@ struct rq {
 #ifdef CONFIG_CFS_BANDWIDTH
 	call_single_data_t	cfsb_csd;
 	struct list_head	cfsb_csd_list;
+	/* cfs_rq->runtime_enabled is in sync with the quota of every group */
+	bool			cfsb_runtime_valid;
 #endif
 
 	atomic_t		nr_iowait;

-- 
2.43.0