sched: improve affine wakeups

improve affine wakeups. Maintain the 'overlap' metric based on CFS's
sum_exec_runtime - which means the amount of time a task executes
after it wakes up some other task.

Use the 'overlap' for the wakeup decisions: if the 'overlap' is short,
it means there's strong workload coupling between this task and the
woken up task. If the 'overlap' is large then the workload is decoupled
and the scheduler will move them to separate CPUs more easily.

( Also slightly move the preempt_check within try_to_wake_up() - this has
  no effect on functionality but allows 'early wakeups' (for still-on-rq
  tasks) to be correctly accounted as well.)

Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/sched.h

@@ -929,6 +929,9 @@ struct sched_entity {
 	u64			vruntime;
 	u64			prev_sum_exec_runtime;
 
+	u64			last_wakeup;
+	u64			avg_overlap;
+
 #ifdef CONFIG_SCHEDSTATS
 	u64			wait_start;
 	u64			wait_max;

kernel/sched.c

@@ -1855,10 +1855,11 @@ out_activate:
 		schedstat_inc(p, se.nr_wakeups_remote);
 	update_rq_clock(rq);
 	activate_task(rq, p, 1);
-	check_preempt_curr(rq, p);
 	success = 1;
 
 out_running:
+	check_preempt_curr(rq, p);
+
 	p->state = TASK_RUNNING;
 #ifdef CONFIG_SMP
 	if (p->sched_class->task_wake_up)
@@ -1892,6 +1893,8 @@ static void __sched_fork(struct task_struct *p)
 	p->se.exec_start		= 0;
 	p->se.sum_exec_runtime		= 0;
 	p->se.prev_sum_exec_runtime	= 0;
+	p->se.last_wakeup		= 0;
+	p->se.avg_overlap		= 0;
 
 #ifdef CONFIG_SCHEDSTATS
 	p->se.wait_start		= 0;

kernel/sched_debug.c

@@ -288,6 +288,7 @@ void proc_sched_show_task(struct task_struct *p, struct seq_file *m)
 	PN(se.exec_start);
 	PN(se.vruntime);
 	PN(se.sum_exec_runtime);
+	PN(se.avg_overlap);
 
 	nr_switches = p->nvcsw + p->nivcsw;
 

kernel/sched_fair.c

@@ -556,6 +556,21 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int wakeup)
 	account_entity_enqueue(cfs_rq, se);
 }
 
+static void update_avg(u64 *avg, u64 sample)
+{
+	s64 diff = sample - *avg;
+	*avg += diff >> 3;
+}
+
+static void update_avg_stats(struct cfs_rq *cfs_rq, struct sched_entity *se)
+{
+	if (!se->last_wakeup)
+		return;
+
+	update_avg(&se->avg_overlap, se->sum_exec_runtime - se->last_wakeup);
+	se->last_wakeup = 0;
+}
+
 static void
 dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int sleep)
 {
@@ -566,6 +581,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int sleep)
 
 	update_stats_dequeue(cfs_rq, se);
 	if (sleep) {
+		update_avg_stats(cfs_rq, se);
 #ifdef CONFIG_SCHEDSTATS
 		if (entity_is_task(se)) {
 			struct task_struct *tsk = task_of(se);
@@ -981,12 +997,15 @@ static inline int wake_idle(int cpu, struct task_struct *p)
 
 #ifdef CONFIG_SMP
 
+static const struct sched_class fair_sched_class;
+
 static int
-wake_affine(struct rq *rq, struct sched_domain *this_sd, struct task_struct *p,
-	    int prev_cpu, int this_cpu, int sync, int idx,
-	    unsigned long load, unsigned long this_load,
+wake_affine(struct rq *rq, struct sched_domain *this_sd, struct rq *this_rq,
+	    struct task_struct *p, int prev_cpu, int this_cpu, int sync,
+	    int idx, unsigned long load, unsigned long this_load,
 	    unsigned int imbalance)
 {
+	struct task_struct *curr = this_rq->curr;
 	unsigned long tl = this_load;
 	unsigned long tl_per_task;
 
@@ -994,10 +1013,15 @@ wake_affine(struct rq *rq, struct sched_domain *this_sd, struct task_struct *p,
 		return 0;
 
 	/*
-	 * Attract cache-cold tasks on sync wakeups:
+	 * If the currently running task will sleep within
+	 * a reasonable amount of time then attract this newly
+	 * woken task:
 	 */
-	if (sync && !task_hot(p, rq->clock, this_sd))
-		return 1;
+	if (sync && curr->sched_class == &fair_sched_class) {
+		if (curr->se.avg_overlap < sysctl_sched_migration_cost &&
+				p->se.avg_overlap < sysctl_sched_migration_cost)
+			return 1;
+	}
 
 	schedstat_inc(p, se.nr_wakeups_affine_attempts);
 	tl_per_task = cpu_avg_load_per_task(this_cpu);
@@ -1030,18 +1054,16 @@ static int select_task_rq_fair(struct task_struct *p, int sync)
 	struct sched_domain *sd, *this_sd = NULL;
 	int prev_cpu, this_cpu, new_cpu;
 	unsigned long load, this_load;
+	struct rq *rq, *this_rq;
 	unsigned int imbalance;
-	struct rq *rq;
 	int idx;
 
 	prev_cpu	= task_cpu(p);
 	rq		= task_rq(p);
 	this_cpu	= smp_processor_id();
+	this_rq		= cpu_rq(this_cpu);
 	new_cpu		= prev_cpu;
 
-	if (prev_cpu == this_cpu)
-		goto out;
-
 	/*
 	 * 'this_sd' is the first domain that both
 	 * this_cpu and prev_cpu are present in:
@@ -1069,11 +1091,12 @@ static int select_task_rq_fair(struct task_struct *p, int sync)
 	load = source_load(prev_cpu, idx);
 	this_load = target_load(this_cpu, idx);
 
-	if (wake_affine(rq, this_sd, p, prev_cpu, this_cpu, sync, idx,
-				     load, this_load, imbalance)) {
-		new_cpu = this_cpu;
+	if (wake_affine(rq, this_sd, this_rq, p, prev_cpu, this_cpu, sync, idx,
+				     load, this_load, imbalance))
+		return this_cpu;
+
+	if (prev_cpu == this_cpu)
 		goto out;
-	}
 
 	/*
 	 * Start passive balancing when half the imbalance_pct
@@ -1083,8 +1106,7 @@ static int select_task_rq_fair(struct task_struct *p, int sync)
 		if (imbalance*this_load <= 100*load) {
 			schedstat_inc(this_sd, ttwu_move_balance);
 			schedstat_inc(p, se.nr_wakeups_passive);
-			new_cpu = this_cpu;
-			goto out;
+			return this_cpu;
 		}
 	}
 
@@ -1111,6 +1133,10 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p)
 		return;
 	}
 
+	se->last_wakeup = se->sum_exec_runtime;
+	if (unlikely(se == pse))
+		return;
+
 	cfs_rq_of(pse)->next = pse;
 
 	/*