[PATCH v2 5/6] sched/rt: Better manage pushing unfit tasks on wakeup

[Qais Yousef]

On wakeup, if a task doesn't fit the CPU it is running on (due to its
uclamp_min value), then we trigger the push mechanism to try to find a
more suitable CPU.

But the logic introduced in commit 804d402fb6f6 ("sched/rt: Make RT capacity-aware")
was incomplete. If the rq isn't overloaded, push_rt_task() will bail out
immediately.

Steve suggested using the overloaded flag to force the push, but as
Pavan pointed out this could cause a lot of unnecessary IPIs in case of
HAVE_RT_PUSH_IPI.

To still allow pushing unfitting task ASAP, but without causing a lot of
disturbance in case this is not possible (no available CPU that is
running at a lower priority level), introduce a new rt_nr_unfitting in
struct rt_rq and use that to manage how hard we try to push an unfitting
task in push_rt_task().

If the task is pinned to a single CPU, we won't inc rt_nr_unfitting,
hence skipping the push in this case.

Also there's no need to force a push on switched_to_rt(). On the next
wakeup we should handle it which should suffice.

Fixes: 804d402fb6f6 ("sched/rt: Make RT capacity-aware")
LINK: https://lore.kernel.org/lkml/20200221080701.GF28029@xxxxxxxxxxxxxx/
Signed-off-by: Qais Yousef <qais.yousef@xxxxxxx>
---
 kernel/sched/rt.c    | 100 +++++++++++++++++++++++++++++++++++++++----
 kernel/sched/sched.h |   3 ++
 2 files changed, 95 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 9ae8a9fabe8b..b35e49cdafcc 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -11,6 +11,7 @@ int sched_rr_timeslice = RR_TIMESLICE;
 int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE;
 
 static int do_sched_rt_period_timer(struct rt_bandwidth *rt_b, int overrun);
+static bool rt_task_fits_capacity(struct task_struct *p, int cpu);
 
 struct rt_bandwidth def_rt_bandwidth;
 
@@ -313,6 +314,27 @@ static void update_rt_migration(struct rt_rq *rt_rq)
 	}
 }
 
+#ifdef CONFIG_UCLAMP_TASK
+static void inc_rt_unfit_tasks(struct task_struct *p, struct rt_rq *rt_rq)
+{
+	int cpu = cpu_of(rq_of_rt_rq(rt_rq));
+
+	if (!rt_task_fits_capacity(p, cpu))
+		rt_rq->rt_nr_unfit++;
+}
+
+static void dec_rt_unfit_tasks(struct task_struct *p, struct rt_rq *rt_rq)
+{
+	int cpu = cpu_of(rq_of_rt_rq(rt_rq));
+
+	if (!rt_task_fits_capacity(p, cpu))
+		rt_rq->rt_nr_unfit--;
+}
+#else
+static void inc_rt_unfit_tasks(struct task_struct *p, struct rt_rq *rt_rq) {}
+static void dec_rt_unfit_tasks(struct task_struct *p, struct rt_rq *rt_rq) {}
+#endif
+
 static void inc_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
 {
 	struct task_struct *p;
@@ -324,9 +346,17 @@ static void inc_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
 	rt_rq = &rq_of_rt_rq(rt_rq)->rt;
 
 	rt_rq->rt_nr_total++;
-	if (p->nr_cpus_allowed > 1)
+	if (p->nr_cpus_allowed > 1) {
 		rt_rq->rt_nr_migratory++;
 
+		/*
+		 * The task is dequeued and queue again on set_cpus_allowed(),
+		 * so we can't end up with a unbalanced inc/dec if
+		 * p->nr_cpus_allowed has changed.
+		 */
+		inc_rt_unfit_tasks(p, rt_rq);
+	}
+
 	update_rt_migration(rt_rq);
 }
 
@@ -341,12 +371,29 @@ static void dec_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
 	rt_rq = &rq_of_rt_rq(rt_rq)->rt;
 
 	rt_rq->rt_nr_total--;
-	if (p->nr_cpus_allowed > 1)
+	if (p->nr_cpus_allowed > 1) {
 		rt_rq->rt_nr_migratory--;
 
+		/*
+		 * The task is dequeued and queue again on set_cpus_allowed(),
+		 * so we can't end up with a unbalanced inc/dec if
+		 * p->nr_cpus_allowed has changed.
+		 */
+		dec_rt_unfit_tasks(p, rt_rq);
+	}
+
 	update_rt_migration(rt_rq);
 }
 
+static inline int has_unfit_tasks(struct rq *rq)
+{
+#ifdef CONFIG_UCLAMP_TASK
+	return rq->rt.rt_nr_unfit;
+#else
+	return 0;
+#endif
+}
+
 static inline int has_pushable_tasks(struct rq *rq)
 {
 	return !plist_head_empty(&rq->rt.pushable_tasks);
@@ -1862,8 +1909,9 @@ static int push_rt_task(struct rq *rq)
 	struct task_struct *next_task;
 	struct rq *lowest_rq;
 	int ret = 0;
+	bool fit;
 
-	if (!rq->rt.overloaded)
+	if (!rq->rt.overloaded && !has_unfit_tasks(rq))
 		return 0;
 
 	next_task = pick_next_pushable_task(rq);
@@ -1874,12 +1922,21 @@ static int push_rt_task(struct rq *rq)
 	if (WARN_ON(next_task == rq->curr))
 		return 0;
 
+	/*
+	 * The rq could be overloaded because it has unfitting task, if that's
+	 * the case then we need to try harder to find a better fitting CPU.
+	 */
+	fit = rt_task_fits_capacity(next_task, cpu_of(rq));
+
 	/*
 	 * It's possible that the next_task slipped in of
 	 * higher priority than current. If that's the case
 	 * just reschedule current.
+	 *
+	 * Unless next_task doesn't fit in this cpu, then continue with the
+	 * attempt to push it.
 	 */
-	if (unlikely(next_task->prio < rq->curr->prio)) {
+	if (unlikely(next_task->prio < rq->curr->prio && fit)) {
 		resched_curr(rq);
 		return 0;
 	}
@@ -1922,6 +1979,35 @@ static int push_rt_task(struct rq *rq)
 		goto retry;
 	}
 
+	/*
+	 * Bail out if the task doesn't fit on either CPUs.
+	 *
+	 * Unless..
+	 *
+	 * * The rq is already overloaded, then push anyway.
+	 *
+	 * * The priority of next_task is higher than current, then we
+	 *   resched_curr(). We forced skipping this condition above if the rq
+	 *   was overloaded but the task didn't fit.
+	 */
+	if (!fit && !rt_task_fits_capacity(next_task, cpu_of(lowest_rq))) {
+
+		/*
+		 * If the system wasn't overloaded, then pretend we didn't run.
+		 */
+		if (!rq->rt.overloaded)
+			goto out_unlock;
+
+		/*
+		 * If the system is overloaded, we forced skipping this
+		 * condition, so re-evaluate it.
+		 */
+		if (unlikely(next_task->prio < rq->curr->prio)) {
+			resched_curr(rq);
+			goto out_unlock;
+		}
+	}
+
 	deactivate_task(rq, next_task, 0);
 	set_task_cpu(next_task, lowest_rq->cpu);
 	activate_task(lowest_rq, next_task, 0);
@@ -1929,6 +2015,7 @@ static int push_rt_task(struct rq *rq)
 
 	resched_curr(lowest_rq);
 
+out_unlock:
 	double_unlock_balance(rq, lowest_rq);
 
 out:
@@ -2297,10 +2384,7 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p)
 	 */
 	if (task_on_rq_queued(p) && rq->curr != p) {
 #ifdef CONFIG_SMP
-		bool need_to_push = rq->rt.overloaded ||
-				    !rt_task_fits_capacity(p, cpu_of(rq));
-
-		if (p->nr_cpus_allowed > 1 && need_to_push)
+		if (p->nr_cpus_allowed > 1 && rq->rt.overloaded)
 			rt_queue_push_tasks(rq);
 #endif /* CONFIG_SMP */
 		if (p->prio < rq->curr->prio && cpu_online(cpu_of(rq)))
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 1a88dc8ad11b..7dea81ccd49a 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -603,6 +603,9 @@ struct rt_rq {
 #ifdef CONFIG_SMP
 	unsigned long		rt_nr_migratory;
 	unsigned long		rt_nr_total;
+#ifdef CONFIG_UCLAMP_TASK
+	unsigned long		rt_nr_unfit;
+#endif
 	int			overloaded;
 	struct plist_head	pushable_tasks;
 
-- 
2.17.1