Message-Id: <20240830130309.2141697-6-vincent.guittot@linaro.org>
Date: Fri, 30 Aug 2024 15:03:09 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: mingo@...hat.com,
	peterz@...radead.org,
	juri.lelli@...hat.com,
	dietmar.eggemann@....com,
	rostedt@...dmis.org,
	bsegall@...gle.com,
	mgorman@...e.de,
	vschneid@...hat.com,
	lukasz.luba@....com,
	rafael.j.wysocki@...el.com,
	linux-kernel@...r.kernel.org
Cc: qyousef@...alina.io,
	hongyan.xia2@....com,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: [RFC PATCH 5/5] sched/fair: Add push task callback for EAS

EAS is based on wakeup events to efficiently place tasks on the system, but
there are cases where a task will not have wakeup events anymore or at a
far too low pace. For such situation, we can take advantage of the task
being put back in the enqueued list to check if it should be migrated on
another CPU. When the task is the only one running on the CPU, the tick
will check it the task is stuck on this CPU and should migrate on another
one.

Wake up events remain the main way to migrate tasks but we now detect
situation where a task is stuck on a CPU by checking that its utilization
is larger than the max available compute capacity (max cpu capacity or
uclamp max setting)

Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
---
 kernel/sched/fair.c  | 211 +++++++++++++++++++++++++++++++++++++++++++
 kernel/sched/sched.h |   2 +
 2 files changed, 213 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e46af2416159..41fb18ac118b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5455,6 +5455,7 @@ static void clear_buddies(struct cfs_rq *cfs_rq, struct sched_entity *se)
 }
 
 static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq);
+static void dequeue_pushable_task(struct cfs_rq *cfs_rq, struct sched_entity *se, bool queue);
 
 static bool
 dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
@@ -5463,6 +5464,8 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 
 	update_curr(cfs_rq);
 
+	dequeue_pushable_task(cfs_rq, se, false);
+
 	if (flags & DEQUEUE_DELAYED) {
 		SCHED_WARN_ON(!se->sched_delayed);
 	} else {
@@ -5585,6 +5588,8 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
 	}
 
 	se->prev_sum_exec_runtime = se->sum_exec_runtime;
+
+	dequeue_pushable_task(cfs_rq, se, true);
 }
 
 static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags);
@@ -5620,6 +5625,7 @@ pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq)
 }
 
 static bool check_cfs_rq_runtime(struct cfs_rq *cfs_rq);
+static void enqueue_pushable_task(struct cfs_rq *cfs_rq, struct sched_entity *se);
 
 static void put_prev_entity(struct cfs_rq *cfs_rq, struct sched_entity *prev)
 {
@@ -5639,9 +5645,16 @@ static void put_prev_entity(struct cfs_rq *cfs_rq, struct sched_entity *prev)
 		__enqueue_entity(cfs_rq, prev);
 		/* in !on_rq case, update occurred at dequeue */
 		update_load_avg(cfs_rq, prev, 0);
+
+		/*
+		 * The previous task might be eligible for pushing it on
+		 * another cpu if it is still active.
+		 */
+		enqueue_pushable_task(cfs_rq, prev);
 	}
 	SCHED_WARN_ON(cfs_rq->curr != prev);
 	cfs_rq->curr = NULL;
+
 }
 
 static void
@@ -8393,6 +8406,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 			target_stat.runnable = cpu_runnable(cpu_rq(cpu));
 			target_stat.capa = capacity_of(cpu);
 			target_stat.nr_running = cpu_rq(cpu)->cfs.h_nr_running;
+			if ((p->on_rq) && (cpu == prev_cpu))
+				target_stat.nr_running--;
 
 			/* If the target needs a lower OPP, then look up for
 			 * the corresponding OPP and its associated cost.
@@ -8473,6 +8488,197 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 	return target;
 }
 
+static inline bool task_misfit_cpu(struct task_struct *p, int cpu)
+{
+	unsigned long max_capa = get_actual_cpu_capacity(cpu);
+	unsigned long util = task_util_est(p);
+
+	max_capa = min(max_capa, uclamp_eff_value(p, UCLAMP_MAX));
+	util = max(util, task_runnable(p));
+
+	/*
+	 * Return true only if the task might not sleep/wakeup because of a low
+	 * compute capacity. Tasks, which wake up regularly, will be handled by
+	 * feec().
+	 */
+	return (util > max_capa);
+}
+
+static int active_load_balance_cpu_stop(void *data);
+
+static inline void check_misfit_cpu(struct task_struct *p, struct rq *rq)
+{
+	int new_cpu, cpu = cpu_of(rq);
+
+	if (!sched_energy_enabled())
+		return;
+
+	if (WARN_ON(!p))
+		return;
+
+	if (WARN_ON(p != rq->curr))
+		return;
+
+	if (is_migration_disabled(p))
+		return;
+
+	if ((rq->nr_running > 1) || (p->nr_cpus_allowed == 1))
+		return;
+
+	if (!task_misfit_cpu(p, cpu))
+		return;
+
+	new_cpu = find_energy_efficient_cpu(p, cpu);
+
+	if (new_cpu == cpu)
+		return;
+
+	/*
+	 * ->active_balance synchronizes accesses to
+	 * ->active_balance_work.  Once set, it's cleared
+	 * only after active load balance is finished.
+	 */
+	if (!rq->active_balance) {
+		rq->active_balance = 1;
+		rq->push_cpu = new_cpu;
+	} else
+		return;
+
+	raw_spin_rq_unlock(rq);
+	stop_one_cpu_nowait(cpu,
+		active_load_balance_cpu_stop, rq,
+		&rq->active_balance_work);
+	raw_spin_rq_lock(rq);
+}
+
+static inline int has_pushable_tasks(struct rq *rq)
+{
+	return !plist_head_empty(&rq->cfs.pushable_tasks);
+}
+
+static struct task_struct *pick_next_pushable_fair_task(struct rq *rq)
+{
+	struct task_struct *p;
+
+	if (!has_pushable_tasks(rq))
+		return NULL;
+
+	p = plist_first_entry(&rq->cfs.pushable_tasks,
+			      struct task_struct, pushable_tasks);
+
+	WARN_ON_ONCE(rq->cpu != task_cpu(p));
+	WARN_ON_ONCE(task_current(rq, p));
+	WARN_ON_ONCE(p->nr_cpus_allowed <= 1);
+
+	WARN_ON_ONCE(!task_on_rq_queued(p));
+
+	/*
+	 * Remove task from the pushable list as we try only once after
+	 * task has been put back in enqueued list.
+	 */
+	plist_del(&p->pushable_tasks, &rq->cfs.pushable_tasks);
+
+	return p;
+}
+
+/*
+ * See if the non running fair tasks on this rq
+ * can be sent to some other CPU that fits better with
+ * their profile.
+ */
+static int push_fair_task(struct rq *rq)
+{
+	struct task_struct *next_task;
+	struct rq *new_rq;
+	int prev_cpu, new_cpu;
+	int ret = 0;
+
+	next_task = pick_next_pushable_fair_task(rq);
+	if (!next_task)
+		return 0;
+
+	if (is_migration_disabled(next_task))
+		return 0;
+
+	if (WARN_ON(next_task == rq->curr))
+		return 0;
+
+	/* We might release rq lock */
+	get_task_struct(next_task);
+
+	prev_cpu = rq->cpu;
+
+	new_cpu = find_energy_efficient_cpu(next_task, prev_cpu);
+
+	if (new_cpu == prev_cpu)
+		goto out;
+
+	new_rq = cpu_rq(new_cpu);
+
+	if (double_lock_balance(rq, new_rq)) {
+
+		deactivate_task(rq, next_task, 0);
+		set_task_cpu(next_task, new_cpu);
+		activate_task(new_rq, next_task, 0);
+		ret = 1;
+
+		resched_curr(new_rq);
+
+		double_unlock_balance(rq, new_rq);
+	}
+
+out:
+	put_task_struct(next_task);
+
+	return ret;
+}
+
+static void push_fair_tasks(struct rq *rq)
+{
+	/* push_dl_task() will return true if it moved a -deadline task */
+	while (push_fair_task(rq))
+		;
+}
+
+static DEFINE_PER_CPU(struct balance_callback, fair_push_head);
+
+static inline void fair_queue_push_tasks(struct rq *rq)
+{
+	if (!sched_energy_enabled() || !has_pushable_tasks(rq))
+		return;
+
+	queue_balance_callback(rq, &per_cpu(fair_push_head, rq->cpu), push_fair_tasks);
+}
+static void dequeue_pushable_task(struct cfs_rq *cfs_rq, struct sched_entity *se, bool queue)
+{
+	struct task_struct *p;
+	struct rq *rq;
+
+	if (sched_energy_enabled() && entity_is_task(se)) {
+		rq = rq_of(cfs_rq);
+		p = container_of(se, struct task_struct, se);
+
+		plist_del(&p->pushable_tasks, &rq->cfs.pushable_tasks);
+
+		if (queue)
+			fair_queue_push_tasks(rq);
+	}
+}
+
+static void enqueue_pushable_task(struct cfs_rq *cfs_rq, struct sched_entity *se)
+{
+	if (sched_energy_enabled() && entity_is_task(se)) {
+		struct task_struct *p = container_of(se, struct task_struct, se);
+		struct rq *rq = rq_of(cfs_rq);
+
+		if ((p->nr_cpus_allowed > 1) && task_misfit_cpu(p, rq->cpu)) {
+			plist_del(&p->pushable_tasks, &rq->cfs.pushable_tasks);
+			plist_node_init(&p->pushable_tasks, p->prio);
+			plist_add(&p->pushable_tasks, &rq->cfs.pushable_tasks);
+		}
+	}
+}
+
 /*
  * select_task_rq_fair: Select target runqueue for the waking task in domains
  * that have the relevant SD flag set. In practice, this is SD_BALANCE_WAKE,
@@ -8642,6 +8848,8 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 	return sched_balance_newidle(rq, rf) != 0;
 }
 #else
+static inline void dequeue_pushable_task(struct cfs_rq *cfs_rq, struct sched_entity *se, bool queue) {}
+static inline void enqueue_pushable_task(struct cfs_rq *cfs_rq, struct sched_entity *se) {}
 static inline void set_task_max_allowed_capacity(struct task_struct *p) {}
 #endif /* CONFIG_SMP */
 
@@ -13013,6 +13221,8 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
 	check_update_overutilized_status(task_rq(curr));
 
 	task_tick_core(rq, curr);
+
+	check_misfit_cpu(curr, rq);
 }
 
 /*
@@ -13204,6 +13414,7 @@ static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first)
 void init_cfs_rq(struct cfs_rq *cfs_rq)
 {
 	cfs_rq->tasks_timeline = RB_ROOT_CACHED;
+	plist_head_init(&cfs_rq->pushable_tasks);
 	cfs_rq->min_vruntime = (u64)(-(1LL << 20));
 #ifdef CONFIG_SMP
 	raw_spin_lock_init(&cfs_rq->removed.lock);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 2f5d658c0631..f3327695d4a3 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -672,6 +672,8 @@ struct cfs_rq {
 	struct list_head	leaf_cfs_rq_list;
 	struct task_group	*tg;	/* group that "owns" this runqueue */
 
+	struct plist_head	pushable_tasks;
+
 	/* Locally cached copy of our task_group's idle value */
 	int			idle;
 
-- 
2.34.1

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