From da5f3cd37802e4d68861422ba4503a4e2d58aa23 Mon Sep 17 00:00:00 2001
From: "J. Avila" <elavila@google.com>
Date: Tue, 14 Jul 2020 22:03:38 +0000
Subject: [PATCH] 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.

Bug: 157906395
Change-Id: I48864d0847bbe4f7813c842032880ad3f3b8b06b
Signed-off-by: J. Avila <elavila@google.com>
---
 kernel/sched/core.c  |  3 +++
 kernel/sched/rt.c    | 39 ++++++++++++++++++++++++++++++++++++++-
 kernel/sched/sched.h |  2 ++
 3 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 70682a97194c..7521eace6e55 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2627,6 +2627,9 @@ 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_held(&rq->lock);
 
 	if (p->sched_contributes_to_load)
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 925cf98d86af..b91f3af14677 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1372,6 +1372,27 @@ static void dequeue_rt_entity(struct sched_rt_entity *rt_se, unsigned int flags)
 	enqueue_top_rt_rq(&rq->rt);
 }
 
+#if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
+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:
  */
@@ -1379,13 +1400,15 @@ 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;
 
 	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);
 }
 
@@ -1462,9 +1485,12 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 {
 	struct task_struct *curr;
 	struct rq *rq;
+	struct rq *this_cpu_rq;
 	bool test;
 	int target_cpu = -1;
 	bool may_not_preempt;
+	bool sync = !!(flags & WF_SYNC);
+	int this_cpu;
 
 	trace_android_rvh_select_task_rq_rt(p, cpu, sd_flag,
 					flags, &target_cpu);
@@ -1479,6 +1505,8 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, 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 a softirq task,
@@ -1516,6 +1544,15 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, 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_capacity(p, cpu)) {
 		int target = find_lowest_rq(p);
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 1a072b6cec1c..0fdc2282ad4a 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1785,6 +1785,8 @@ extern const u32		sched_prio_to_wmult[40];
 #define ENQUEUE_MIGRATED	0x00
 #endif
 
+#define ENQUEUE_WAKEUP_SYNC	0x80
+
 #define RETRY_TASK		((void *)-1UL)
 
 struct sched_class {