[PATCH 01/11] 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
Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
---
 kernel/sched/core.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3cc6fb1d20547..f47bcfb25033a 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;
 	}
 
-- 
2.55.0