[PATCH 0/2] sched/fair: Cut per-group work from sched domain rebuilds

[PATCH 0/2] sched/fair: Cut per-group work from sched domain rebuilds

[Qiliang Yuan]

A sched domain rebuild, from CPU hotplug or a cpuset partition change,
moves every active CPU to a new root domain through rq_attach_root().
For each of them rq_offline_fair() and rq_online_fair() walk all task
groups with the rq lock held and interrupts disabled, so a rebuild
costs O(nr_cpus * nr_task_groups). Dietmar pointed at the offline side
when f60a631ab9ed was reviewed [1].

Patch 1 skips rq_offline_fair() for CPUs that stay active. Patch 2
skips the runtime_enabled re-sync in rq_online_fair() unless the CPU
actually went down.

With 4000 cpu cgroups on a 32-CPU VM booted with rt_group_sched=0
(median of 20 partition switches and 16 hotplug operations, all three
kernels measured in the same run):

  operation         base (ms)  patch 1 (ms)  patch 1+2 (ms)  change
  partition root        13.28          7.68            1.21    -91%
  cpu offline           44.95         30.96           13.44    -70%
  cpu online            36.84         28.48           12.82    -65%

With both patches these times no longer grow with the number of
groups. With RT_GROUP_SCHED enabled, the rt.c runtime walks remain.

[1] https://lore.kernel.org/r/dbd36b16-4acb-415a-9bbd-258b2e022475@arm.com

Signed-off-by: Qiliang Yuan <qiliang.yuan@linux.dev>
---
Qiliang Yuan (2):
      sched/fair: Skip rq_offline_fair() work for CPUs that stay active
      sched/fair: Avoid re-syncing runtime_enabled on root domain changes

 kernel/sched/fair.c  | 20 +++++++++++++++++---
 kernel/sched/sched.h |  2 ++
 2 files changed, 19 insertions(+), 3 deletions(-)
---
base-commit: 067c5c24cef23d913b67eaaf71ae67e7169a6620
change-id: 20261111-bug-sched-fair-offline-active-8d063e735d3f

Best regards,
-- 
Qiliang Yuan <qiliang.yuan@linux.dev>

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

[Qiliang Yuan]

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

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

[Qiliang Yuan]

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