sched/fair: Block nohz tick_stop when cfs bandwidth in use

CFS bandwidth limits and NOHZ full don't play well together.  Tasks
can easily run well past their quotas before a remote tick does
accounting.  This leads to long, multi-period stalls before such
tasks can run again. Currently, when presented with these conflicting
requirements the scheduler is favoring nohz_full and letting the tick
be stopped. However, nohz tick stopping is already best-effort, there
are a number of conditions that can prevent it, whereas cfs runtime
bandwidth is expected to be enforced.

Make the scheduler favor bandwidth over stopping the tick by setting
TICK_DEP_BIT_SCHED when the only running task is a cfs task with
runtime limit enabled. We use cfs_b->hierarchical_quota to
determine if the task requires the tick.

Add check in pick_next_task_fair() as well since that is where
we have a handle on the task that is actually going to be running.

Add check in sched_can_stop_tick() to cover some edge cases such
as nr_running going from 2->1 and the 1 remains the running task.

Reviewed-By: Ben Segall <bsegall@google.com>
Signed-off-by: Phil Auld <pauld@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230712133357.381137-3-pauld@redhat.com

1 files changed, 26 insertions, 0 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3af25caf6343..614271a75525 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1194,6 +1194,20 @@ 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)
+{
+	if (rq->nr_running != 1)
+		return false;
+
+	if (p->sched_class != &fair_sched_class)
+		return false;
+
+	if (!task_on_rq_queued(p))
+		return false;
+
+	return true;
+}
+
 bool sched_can_stop_tick(struct rq *rq)
 {
 	int fifo_nr_running;
@@ -1229,6 +1243,18 @@ bool sched_can_stop_tick(struct rq *rq)
 	if (rq->nr_running > 1)
 		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.
+	 * This check prevents clearing the bit if a newly enqueued task here is
+	 * dequeued by migrating while the constrained task continues to run.
+	 * E.g. going from 2->1 without going through pick_next_task().
+	 */
+	if (sched_feat(HZ_BW) && __need_bw_check(rq, rq->curr)) {
+		if (cfs_task_bw_constrained(rq->curr))
+			return false;
+	}
+
 	return true;
 }
 #endif /* CONFIG_NO_HZ_FULL */