ANDROID: sched/rt: schedtune: Add boost retention to RT

Boosted RT tasks can be deboosted quickly, this makes boost usless
for RT tasks and causes lots of glitching. Use timers to prevent
de-boost too soon and wait for long enough such that next enqueue
happens after a threshold.

While this can be solved in the governor, there are following
advantages:
- The approach used is governor-independent
- Reduces boost group lock contention for frequently sleepers/wakers
- Works with schedfreq without any other schedfreq hacks.

Bug: 30210506

Change-Id: I41788b235586988be446505deb7c0529758a9898
Signed-off-by: Joel Fernandes <joelaf@google.com>

Author: Joel Fernandes

include/linux/sched.h

@@ -1433,6 +1433,10 @@ struct sched_rt_entity {
 	unsigned long watchdog_stamp;
 	unsigned int time_slice;
 
+	/* Accesses for these must be guarded by rq->lock of the task's rq */
+	bool schedtune_enqueued;
+	struct hrtimer schedtune_timer;
+
 	struct sched_rt_entity *back;
 #ifdef CONFIG_RT_GROUP_SCHED
 	struct sched_rt_entity	*parent;

kernel/sched/core.c

@@ -2200,6 +2200,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
 	init_dl_task_timer(&p->dl);
 	__dl_clear_params(p);
 
+	init_rt_schedtune_timer(&p->rt);
 	INIT_LIST_HEAD(&p->rt.run_list);
 
 #ifdef CONFIG_PREEMPT_NOTIFIERS

kernel/sched/rt.c

@@ -7,6 +7,7 @@
 
 #include <linux/slab.h>
 #include <linux/irq_work.h>
+#include <linux/hrtimer.h>
 
 #include "walt.h"
 #include "tune.h"
@@ -986,6 +987,73 @@ static int sched_rt_runtime_exceeded(struct rt_rq *rt_rq)
 	return 0;
 }
 
+#define RT_SCHEDTUNE_INTERVAL 50000000ULL
+
+static void sched_rt_update_capacity_req(struct rq *rq);
+
+static enum hrtimer_restart rt_schedtune_timer(struct hrtimer *timer)
+{
+	struct sched_rt_entity *rt_se = container_of(timer,
+			struct sched_rt_entity,
+			schedtune_timer);
+	struct task_struct *p = rt_task_of(rt_se);
+	struct rq *rq = task_rq(p);
+
+	raw_spin_lock(&rq->lock);
+
+	/*
+	 * Nothing to do if:
+	 * - task has switched runqueues
+	 * - task isn't RT anymore
+	 */
+	if (rq != task_rq(p) || (p->sched_class != &rt_sched_class))
+		goto out;
+
+	/*
+	 * If task got enqueued back during callback time, it means we raced
+	 * with the enqueue on another cpu, that's Ok, just do nothing as
+	 * enqueue path would have tried to cancel us and we shouldn't run
+	 * Also check the schedtune_enqueued flag as class-switch on a
+	 * sleeping task may have already canceled the timer and done dq
+	 */
+	if (p->on_rq || !rt_se->schedtune_enqueued)
+		goto out;
+
+	/*
+	 * RT task is no longer active, cancel boost
+	 */
+	rt_se->schedtune_enqueued = false;
+	schedtune_dequeue_task(p, cpu_of(rq));
+	sched_rt_update_capacity_req(rq);
+	cpufreq_update_this_cpu(rq, SCHED_CPUFREQ_RT);
+out:
+	raw_spin_unlock(&rq->lock);
+
+	/*
+	 * This can free the task_struct if no more references.
+	 */
+	put_task_struct(p);
+
+	return HRTIMER_NORESTART;
+}
+
+void init_rt_schedtune_timer(struct sched_rt_entity *rt_se)
+{
+	struct hrtimer *timer = &rt_se->schedtune_timer;
+
+	hrtimer_init(timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	timer->function = rt_schedtune_timer;
+	rt_se->schedtune_enqueued = false;
+}
+
+static void start_schedtune_timer(struct sched_rt_entity *rt_se)
+{
+	struct hrtimer *timer = &rt_se->schedtune_timer;
+
+	hrtimer_start(timer, ns_to_ktime(RT_SCHEDTUNE_INTERVAL),
+			HRTIMER_MODE_REL_PINNED);
+}
+
 /*
  * Update the current task's runtime statistics. Skip current tasks that
  * are not in our scheduling class.
@@ -1323,7 +1391,33 @@ enqueue_task_rt(struct rq *rq, struct task_struct *p, int flags)
 	if (!task_current(rq, p) && p->nr_cpus_allowed > 1)
 		enqueue_pushable_task(rq, p);
 
+	if (!schedtune_task_boost(p))
+		return;
+
+	/*
+	 * If schedtune timer is active, that means a boost was already
+	 * done, just cancel the timer so that deboost doesn't happen.
+	 * Otherwise, increase the boost. If an enqueued timer was
+	 * cancelled, put the task reference.
+	 */
+	if (hrtimer_try_to_cancel(&rt_se->schedtune_timer) == 1)
+		put_task_struct(p);
+
+	/*
+	 * schedtune_enqueued can be true in the following situation:
+	 * enqueue_task_rt grabs rq lock before timer fires
+	 *    or before its callback acquires rq lock
+	 * schedtune_enqueued can be false if timer callback is running
+	 * and timer just released rq lock, or if the timer finished
+	 * running and canceling the boost
+	 */
+	if (rt_se->schedtune_enqueued)
+		return;
+
+	rt_se->schedtune_enqueued = true;
 	schedtune_enqueue_task(p, cpu_of(rq));
+	sched_rt_update_capacity_req(rq);
+	cpufreq_update_this_cpu(rq, SCHED_CPUFREQ_RT);
 }
 
 static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int flags)
@@ -1335,7 +1429,20 @@ static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int flags)
 	walt_dec_cumulative_runnable_avg(rq, p);
 
 	dequeue_pushable_task(rq, p);
+
+	if (!rt_se->schedtune_enqueued)
+		return;
+
+	if (flags == DEQUEUE_SLEEP) {
+		get_task_struct(p);
+		start_schedtune_timer(rt_se);
+		return;
+	}
+
+	rt_se->schedtune_enqueued = false;
 	schedtune_dequeue_task(p, cpu_of(rq));
+	sched_rt_update_capacity_req(rq);
+	cpufreq_update_this_cpu(rq, SCHED_CPUFREQ_RT);
 }
 
 /*
@@ -1375,6 +1482,33 @@ static void yield_task_rt(struct rq *rq)
 #ifdef CONFIG_SMP
 static int find_lowest_rq(struct task_struct *task);
 
+/*
+ * Perform a schedtune dequeue and cancelation of boost timers if needed.
+ * Should be called only with the rq->lock held.
+ */
+static void schedtune_dequeue_rt(struct rq *rq, struct task_struct *p)
+{
+	struct sched_rt_entity *rt_se = &p->rt;
+
+	BUG_ON(!raw_spin_is_locked(&rq->lock));
+
+	if (!rt_se->schedtune_enqueued)
+		return;
+
+	/*
+	 * Incase of class change cancel any active timers. If an enqueued
+	 * timer was cancelled, put the task ref.
+	 */
+	if (hrtimer_try_to_cancel(&rt_se->schedtune_timer) == 1)
+		put_task_struct(p);
+
+	/* schedtune_enqueued is true, deboost it */
+	rt_se->schedtune_enqueued = false;
+	schedtune_dequeue_task(p, task_cpu(p));
+	sched_rt_update_capacity_req(rq);
+	cpufreq_update_this_cpu(rq, SCHED_CPUFREQ_RT);
+}
+
 static int
 select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags,
 		  int sibling_count_hint)
@@ -1429,6 +1563,19 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags,
 	rcu_read_unlock();
 
 out:
+	/*
+	 * If previous CPU was different, make sure to cancel any active
+	 * schedtune timers and deboost.
+	 */
+	if (task_cpu(p) != cpu) {
+		unsigned long fl;
+		struct rq *prq = task_rq(p);
+
+		raw_spin_lock_irqsave(&prq->lock, fl);
+		schedtune_dequeue_rt(prq, p);
+		raw_spin_unlock_irqrestore(&prq->lock, fl);
+	}
+
 	return cpu;
 }
 
@@ -2161,6 +2308,13 @@ static void rq_offline_rt(struct rq *rq)
  */
 static void switched_from_rt(struct rq *rq, struct task_struct *p)
 {
+	/*
+	 * On class switch from rt, always cancel active schedtune timers,
+	 * this handles the cases where we switch class for a task that is
+	 * already rt-dequeued but has a running timer.
+	 */
+	schedtune_dequeue_rt(rq, p);
+
 	/*
 	 * If there are other RT tasks then we will reschedule
 	 * and the scheduling of the other RT tasks will handle

kernel/sched/sched.h

@@ -1398,6 +1398,7 @@ extern void resched_cpu(int cpu);
 
 extern struct rt_bandwidth def_rt_bandwidth;
 extern void init_rt_bandwidth(struct rt_bandwidth *rt_b, u64 period, u64 runtime);
+extern void init_rt_schedtune_timer(struct sched_rt_entity *rt_se);
 
 extern struct dl_bandwidth def_dl_bandwidth;
 extern void init_dl_bandwidth(struct dl_bandwidth *dl_b, u64 period, u64 runtime);