[PATCH 02/14] sched: Make NOHZ CFS bandwidth checks follow proxy donor

[Andrea Righi]

Proxy execution separates the scheduling context in rq->donor from the
physical execution context in rq->curr. sched_can_stop_tick() checks the
latter for CFS bandwidth constraints and only does so when nr_running is
one.

A retained proxy donor keeps both the donor and mutex owner queued. The
check therefore misses a constrained FAIR donor and may stop the tick
while its runtime still needs to be enforced.

Check the selected donor instead and remove the nr_running restriction.
The donor being a queued FAIR task is sufficient to require bandwidth
accounting regardless of other runnable tasks.

Fixes: af0c8b2bf67b ("sched: Split scheduler and execution contexts")
Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20260713164807.E5ED21F00A3A@xxxxxxxxxxxxxxx
Acked-by: John Stultz <jstultz@xxxxxxxxxx>
Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
---
 kernel/sched/core.c | 18 +++++++++---------
 kernel/sched/fair.c | 12 +++++++-----
 2 files changed, 16 insertions(+), 14 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e11fac7f805e7..e9829a289da45 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1409,11 +1409,8 @@ static void nohz_csd_func(void *info)
 #endif /* CONFIG_NO_HZ_COMMON */
 
 #ifdef CONFIG_NO_HZ_FULL
-static inline bool __need_bw_check(struct rq *rq, struct task_struct *p)
+static inline bool __need_bw_check(struct task_struct *p)
 {
-	if (rq->nr_running != 1)
-		return false;
-
 	if (p->sched_class != &fair_sched_class)
 		return false;
 
@@ -1462,14 +1459,17 @@ bool sched_can_stop_tick(struct rq *rq)
 		return false;
 
 	/*
-	 * If there is one task and it has CFS runtime bandwidth constraints
-	 * and it's on the cpu now we don't want to stop the tick.
+	 * If the selected scheduling context has CFS runtime bandwidth
+	 * constraints, don't stop the tick. With proxy execution, rq->donor is
+	 * the selected scheduling context while rq->curr is the task physically
+	 * executing on its behalf.
+	 *
 	 * This check prevents clearing the bit if a newly enqueued task here is
-	 * dequeued by migrating while the constrained task continues to run.
+	 * dequeued by migrating while the constrained donor continues to run.
 	 * E.g. going from 2->1 without going through pick_next_task().
 	 */
-	if (__need_bw_check(rq, rq->curr)) {
-		if (cfs_task_bw_constrained(rq->curr))
+	if (__need_bw_check(rq->donor)) {
+		if (cfs_task_bw_constrained(rq->donor))
 			return false;
 	}
 
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d78467ec6ee13..14c5b2aa01777 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7570,14 +7570,16 @@ static void sched_fair_update_stop_tick(struct rq *rq, struct task_struct *p)
 	if (!tick_nohz_full_cpu(cpu))
 		return;
 
-	if (rq->nr_running != 1)
+	if (rq->cfs.h_nr_queued != 1)
 		return;
 
 	/*
-	 *  We know there is only one task runnable and we've just picked it. The
-	 *  normal enqueue path will have cleared TICK_DEP_BIT_SCHED if we will
-	 *  be otherwise able to stop the tick. Just need to check if we are using
-	 *  bandwidth control.
+	 * We know there is only one FAIR task queued and we've just picked it.
+	 * The normal enqueue path will have cleared TICK_DEP_BIT_SCHED if we
+	 * will be otherwise able to stop the tick. With proxy execution, its
+	 * lock owner may remain runnable in another scheduling class, so
+	 * rq->nr_running can be greater than one. Just need to check if we are
+	 * using bandwidth control.
 	 */
 	if (cfs_task_bw_constrained(p))
 		tick_nohz_dep_set_cpu(cpu, TICK_DEP_BIT_SCHED);
-- 
2.55.0