ANDROID: sched/rt: Add support for rt sync wakeups

Some rt tasks undergo sync wakeup. Currently, these tasks will be placed
on other, often sleeping or otherwise idle CPUs, which can lead to
unnecessary power hits. Support rt sync wakeups, but only enable
rt sync for SMP targets.

Bug: 157906395
Bug: 323845182
Old-Change-Id: I48864d0847bbe4f7813c842032880ad3f3b8b06b
Change-Id: Ia22b2077969c023e6b0d40861626d55420f657ad
Signed-off-by: J. Avila <elavila@google.com>
[quic_dickey@quicinc.com: Port to mainline]
Signed-off-by: Stephen Dickey <quic_dickey@quicinc.com>
[quic_snimmala@quicinc.com: Fixed conflicts in core.c]
Signed-off-by: Sai Harshini Nimmala <quic_snimmala@quicinc.com>
[jstultz: Cherry-picked forward]
Signed-off-by: John Stultz <jstultz@google.com>

kernel/sched/core.c

@@ -3722,6 +3722,9 @@ ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
 {
 	int en_flags = ENQUEUE_WAKEUP | ENQUEUE_NOCLOCK;
 
+	if (wake_flags & WF_SYNC)
+		en_flags |= ENQUEUE_WAKEUP_SYNC;
+
 	lockdep_assert_rq_held(rq);
 
 	if (p->sched_contributes_to_load)

kernel/sched/rt.c

@@ -1472,6 +1472,27 @@ static void dequeue_rt_entity(struct sched_rt_entity *rt_se, unsigned int flags)
 	enqueue_top_rt_rq(&rq->rt);
 }
 
+#ifdef CONFIG_SMP
+static inline bool should_honor_rt_sync(struct rq *rq, struct task_struct *p,
+					bool sync)
+{
+	/*
+	 * If the waker is CFS, then an RT sync wakeup would preempt the waker
+	 * and force it to run for a likely small time after the RT wakee is
+	 * done. So, only honor RT sync wakeups from RT wakers.
+	 */
+	return sync && task_has_rt_policy(rq->curr) &&
+		p->prio <= rq->rt.highest_prio.next &&
+		rq->rt.rt_nr_running <= 2;
+}
+#else
+static inline bool should_honor_rt_sync(struct rq *rq, struct task_struct *p,
+					bool sync)
+{
+	return 0;
+}
+#endif
+
 /*
  * Adding/removing a task to/from a priority array:
  */
@@ -1479,6 +1500,7 @@ static void
 enqueue_task_rt(struct rq *rq, struct task_struct *p, int flags)
 {
 	struct sched_rt_entity *rt_se = &p->rt;
+	bool sync = !!(flags & ENQUEUE_WAKEUP_SYNC);
 
 	if (flags & ENQUEUE_WAKEUP)
 		rt_se->timeout = 0;
@@ -1488,7 +1510,8 @@ enqueue_task_rt(struct rq *rq, struct task_struct *p, int flags)
 
 	enqueue_rt_entity(rt_se, flags);
 
-	if (!task_current(rq, p) && p->nr_cpus_allowed > 1)
+	if (!task_current(rq, p) && p->nr_cpus_allowed > 1 &&
+	    !should_honor_rt_sync(rq, p, sync))
 		enqueue_pushable_task(rq, p);
 }
 
@@ -1572,8 +1595,11 @@ select_task_rq_rt(struct task_struct *p, int cpu, int flags)
 {
 	struct task_struct *curr;
 	struct rq *rq;
+	struct rq *this_cpu_rq;
 	bool test;
 	int target_cpu = -1;
+	bool sync = !!(flags & WF_SYNC);
+	int this_cpu;
 
 	trace_android_rvh_select_task_rq_rt(p, cpu, flags & 0xF,
 					flags, &target_cpu);
@@ -1588,6 +1614,8 @@ select_task_rq_rt(struct task_struct *p, int cpu, int flags)
 
 	rcu_read_lock();
 	curr = READ_ONCE(rq->curr); /* unlocked access */
+	this_cpu = smp_processor_id();
+	this_cpu_rq = cpu_rq(this_cpu);
 
 	/*
 	 * If the current task on @p's runqueue is an RT task, then
@@ -1621,6 +1649,15 @@ select_task_rq_rt(struct task_struct *p, int cpu, int flags)
 	       unlikely(rt_task(curr)) &&
 	       (curr->nr_cpus_allowed < 2 || curr->prio <= p->prio);
 
+	/*
+	 * Respect the sync flag as long as the task can run on this CPU.
+	 */
+	if (should_honor_rt_sync(this_cpu_rq, p, sync) &&
+	    cpumask_test_cpu(this_cpu, p->cpus_ptr)) {
+		cpu = this_cpu;
+		goto out_unlock;
+	}
+
 	if (test || !rt_task_fits_cpu(p, cpu)) {
 		int target = find_lowest_rq(p);
 

kernel/sched/sched.h

@@ -2381,6 +2381,8 @@ extern const u32		sched_prio_to_wmult[40];
 #define ENQUEUE_DELAYED		0x200
 #define ENQUEUE_RQ_SELECTED	0x400
 
+#define ENQUEUE_WAKEUP_SYNC	0x80
+
 #define RETRY_TASK		((void *)-1UL)
 
 struct affinity_context {