[PATCH v3] Sched/fair: Block nohz tick_stop when cfs bandwidth in use

[Phil Auld <pauld@redhat.com>]

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. Currentlyi, 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.

Add sched_feat HZ_BW (off by default) to control this behavior.

Signed-off-by: Phil Auld <pauld@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Ben Segall <bsegall@google.com>
---

v2:  Ben pointed out that the bit could get cleared in the dequeue path
if we migrate a newly enqueued task without preempting curr. Added a
check for that edge case to sched_can_stop_tick. Removed the call to
sched_can_stop_tick from sched_fair_update_stop_tick since it was
redundant.

v3: Moved sched_cfs_bandwidth_active() prototype to sched.h outside of
guards to silence -Wmissing-prototypes.

 kernel/sched/core.c     | 10 +++++++++
 kernel/sched/fair.c     | 45 +++++++++++++++++++++++++++++++++++++++++
 kernel/sched/features.h |  2 ++
 kernel/sched/sched.h    |  1 +
 4 files changed, 58 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a68d1276bab0..0d8bdd36f870 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1229,6 +1229,16 @@ 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.
+	 */
+	if (sched_feat(HZ_BW) && rq->nr_running == 1 && rq->curr
+	    && rq->curr->sched_class == &fair_sched_class && task_on_rq_queued(rq->curr)) {
+		if (sched_cfs_bandwidth_active(task_cfs_rq(rq->curr)))
+			return false;
+	}
+
 	return true;
 }
 #endif /* CONFIG_NO_HZ_FULL */
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 373ff5f55884..a05af33b8da9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6139,6 +6139,42 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
 	rcu_read_unlock();
 }
 
+#ifdef CONFIG_NO_HZ_FULL
+
+bool sched_cfs_bandwidth_active(struct cfs_rq *cfs_rq)
+{
+	if (cfs_bandwidth_used() && cfs_rq->runtime_enabled)
+		return true;
+
+	return false;
+}
+
+/* called from pick_next_task_fair() */
+static void sched_fair_update_stop_tick(struct rq *rq, struct task_struct *p)
+{
+	struct cfs_rq *cfs_rq = task_cfs_rq(p);
+	int cpu = cpu_of(rq);
+
+	if (!sched_feat(HZ_BW) || !cfs_bandwidth_used())
+		return;
+
+	if (!tick_nohz_full_cpu(cpu))
+		return;
+
+	if (rq->nr_running != 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.
+	 */
+	if (cfs_rq->runtime_enabled)
+		tick_nohz_dep_set_cpu(cpu, TICK_DEP_BIT_SCHED);
+}
+#endif
+
 #else /* CONFIG_CFS_BANDWIDTH */
 
 static inline bool cfs_bandwidth_used(void)
@@ -6181,9 +6217,17 @@ static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
 static inline void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b) {}
 static inline void update_runtime_enabled(struct rq *rq) {}
 static inline void unthrottle_offline_cfs_rqs(struct rq *rq) {}
+bool sched_cfs_bandwidth_active(struct cfs_rq *cfs_rq)
+{
+	return false;
+}
 
 #endif /* CONFIG_CFS_BANDWIDTH */
 
+#if !defined(CONFIG_CFS_BANDWIDTH) || !defined(CONFIG_NO_HZ_FULL)
+static inline void sched_fair_update_stop_tick(struct rq *rq, struct task_struct *p) {}
+#endif
+
 /**************************************************
  * CFS operations on tasks:
  */
@@ -8097,6 +8141,7 @@ done: __maybe_unused;
 		hrtick_start_fair(rq, p);
 
 	update_misfit_status(p, rq);
+	sched_fair_update_stop_tick(rq, p);
 
 	return p;
 
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index ee7f23c76bd3..6fdf1fdf6b17 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -101,3 +101,5 @@ SCHED_FEAT(LATENCY_WARN, false)
 
 SCHED_FEAT(ALT_PERIOD, true)
 SCHED_FEAT(BASE_SLICE, true)
+
+SCHED_FEAT(HZ_BW, false)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index ec7b3e0a2b20..f2bd4568472e 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2789,6 +2789,7 @@ extern void init_dl_rq(struct dl_rq *dl_rq);
 
 extern void cfs_bandwidth_usage_inc(void);
 extern void cfs_bandwidth_usage_dec(void);
+extern bool sched_cfs_bandwidth_active(struct cfs_rq *cfs_rq);
 
 #ifdef CONFIG_NO_HZ_COMMON
 #define NOHZ_BALANCE_KICK_BIT	0
-- 
2.31.1