sched: wakeup preempt when small overlap

Lin Ming reported a 10% OLTP regression against 2.6.27-rc4.

The difference seems to come from different preemption agressiveness,
which affects the cache footprint of the workload and its effective
cache trashing.

Aggresively preempt a task if its avg overlap is very small, this should
avoid the task going to sleep and find it still running when we schedule
back to it - saving a wakeup.
Reported-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/sched.h

@@ -897,7 +897,7 @@ struct sched_class {
 	void (*yield_task) (struct rq *rq);
 	int  (*select_task_rq)(struct task_struct *p, int sync);
 
-	void (*check_preempt_curr) (struct rq *rq, struct task_struct *p);
+	void (*check_preempt_curr) (struct rq *rq, struct task_struct *p, int sync);
 
 	struct task_struct * (*pick_next_task) (struct rq *rq);
 	void (*put_prev_task) (struct rq *rq, struct task_struct *p);

kernel/sched.c

@@ -604,9 +604,9 @@ struct rq {
 
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
 
-static inline void check_preempt_curr(struct rq *rq, struct task_struct *p)
+static inline void check_preempt_curr(struct rq *rq, struct task_struct *p, int sync)
 {
-	rq->curr->sched_class->check_preempt_curr(rq, p);
+	rq->curr->sched_class->check_preempt_curr(rq, p, sync);
 }
 
 static inline int cpu_of(struct rq *rq)
@@ -2282,7 +2282,7 @@ static int try_to_wake_up(struct task_struct *p, unsigned int state, int sync)
 	trace_mark(kernel_sched_wakeup,
 		"pid %d state %ld ## rq %p task %p rq->curr %p",
 		p->pid, p->state, rq, p, rq->curr);
-	check_preempt_curr(rq, p);
+	check_preempt_curr(rq, p, sync);
 
 	p->state = TASK_RUNNING;
 #ifdef CONFIG_SMP
@@ -2417,7 +2417,7 @@ void wake_up_new_task(struct task_struct *p, unsigned long clone_flags)
 	trace_mark(kernel_sched_wakeup_new,
 		"pid %d state %ld ## rq %p task %p rq->curr %p",
 		p->pid, p->state, rq, p, rq->curr);
-	check_preempt_curr(rq, p);
+	check_preempt_curr(rq, p, 0);
 #ifdef CONFIG_SMP
 	if (p->sched_class->task_wake_up)
 		p->sched_class->task_wake_up(rq, p);
@@ -2877,7 +2877,7 @@ static void pull_task(struct rq *src_rq, struct task_struct *p,
 	 * Note that idle threads have a prio of MAX_PRIO, for this test
 	 * to be always true for them.
 	 */
-	check_preempt_curr(this_rq, p);
+	check_preempt_curr(this_rq, p, 0);
 }
 
 /*
@@ -6007,7 +6007,7 @@ static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
 	set_task_cpu(p, dest_cpu);
 	if (on_rq) {
 		activate_task(rq_dest, p, 0);
-		check_preempt_curr(rq_dest, p);
+		check_preempt_curr(rq_dest, p, 0);
 	}
 done:
 	ret = 1;

kernel/sched_fair.c

@@ -1331,7 +1331,7 @@ static inline int depth_se(struct sched_entity *se)
 /*
  * Preempt the current task with a newly woken task if needed:
  */
-static void check_preempt_wakeup(struct rq *rq, struct task_struct *p)
+static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int sync)
 {
 	struct task_struct *curr = rq->curr;
 	struct cfs_rq *cfs_rq = task_cfs_rq(curr);
@@ -1367,6 +1367,13 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p)
 	if (!sched_feat(WAKEUP_PREEMPT))
 		return;
 
+	if (sched_feat(WAKEUP_OVERLAP) && sync &&
+			se->avg_overlap < sysctl_sched_migration_cost &&
+			pse->avg_overlap < sysctl_sched_migration_cost) {
+		resched_task(curr);
+		return;
+	}
+
 	/*
 	 * preemption test can be made between sibling entities who are in the
 	 * same cfs_rq i.e who have a common parent. Walk up the hierarchy of
@@ -1649,7 +1656,7 @@ static void prio_changed_fair(struct rq *rq, struct task_struct *p,
 		if (p->prio > oldprio)
 			resched_task(rq->curr);
 	} else
-		check_preempt_curr(rq, p);
+		check_preempt_curr(rq, p, 0);
 }
 
 /*
@@ -1666,7 +1673,7 @@ static void switched_to_fair(struct rq *rq, struct task_struct *p,
 	if (running)
 		resched_task(rq->curr);
 	else
-		check_preempt_curr(rq, p);
+		check_preempt_curr(rq, p, 0);
 }
 
 /* Account for a task changing its policy or group.

kernel/sched_features.h

@@ -11,3 +11,4 @@ SCHED_FEAT(ASYM_GRAN, 1)
 SCHED_FEAT(LB_BIAS, 1)
 SCHED_FEAT(LB_WAKEUP_UPDATE, 1)
 SCHED_FEAT(ASYM_EFF_LOAD, 1)
+SCHED_FEAT(WAKEUP_OVERLAP, 1)

kernel/sched_idletask.c

@@ -14,7 +14,7 @@ static int select_task_rq_idle(struct task_struct *p, int sync)
 /*
  * Idle tasks are unconditionally rescheduled:
  */
-static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p)
+static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int sync)
 {
 	resched_task(rq->idle);
 }
@@ -76,7 +76,7 @@ static void switched_to_idle(struct rq *rq, struct task_struct *p,
 	if (running)
 		resched_task(rq->curr);
 	else
-		check_preempt_curr(rq, p);
+		check_preempt_curr(rq, p, 0);
 }
 
 static void prio_changed_idle(struct rq *rq, struct task_struct *p,
@@ -93,7 +93,7 @@ static void prio_changed_idle(struct rq *rq, struct task_struct *p,
 		if (p->prio > oldprio)
 			resched_task(rq->curr);
 	} else
-		check_preempt_curr(rq, p);
+		check_preempt_curr(rq, p, 0);
 }
 
 /*

kernel/sched_rt.c

@@ -783,7 +783,7 @@ static void check_preempt_equal_prio(struct rq *rq, struct task_struct *p)
 /*
  * Preempt the current task with a newly woken task if needed:
  */
-static void check_preempt_curr_rt(struct rq *rq, struct task_struct *p)
+static void check_preempt_curr_rt(struct rq *rq, struct task_struct *p, int sync)
 {
 	if (p->prio < rq->curr->prio) {
 		resched_task(rq->curr);