[PATCH v4 3/4] mark whenever put_prev_task is called from context_switch

[Glauber Costa]

In the interest of having the number of context switches stored per-group,
this patch introduces a boolean argument to put_prev_task. It is needed
because put_prev_task is called from many sites, not all of them denoting
a context switch.

This would be needed even if pick_next_task has the behavior of put_prev_task
embedded into it: This is because we may be picking a task from a different
class, and that would translate to a raw call to put_prev_task.

For the fair class, nr_switches is stored for all the hierarchy. This is
because a walk is done anyway there, so we just reuse it.

For the rt class, we only store in the current group, therefore not
forcing a walk. Readers then may have to accumulate.

Signed-off-by: Glauber Costa <glommer@xxxxxxxxxxxxx>
CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
CC: Paul Turner <pjt@xxxxxxxxxx>
---
 include/linux/sched.h    |    2 +-
 kernel/sched/core.c      |   11 ++++++-----
 kernel/sched/fair.c      |    4 +++-
 kernel/sched/idle_task.c |    2 +-
 kernel/sched/rt.c        |    6 +++++-
 kernel/sched/sched.h     |    3 +++
 kernel/sched/stop_task.c |    2 +-
 7 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index f45c0b2..2bff097 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1083,7 +1083,7 @@ struct sched_class {
 	void (*check_preempt_curr) (struct rq *rq, struct task_struct *p, int flags);
 
 	struct task_struct * (*pick_next_task) (struct rq *rq);
-	void (*put_prev_task) (struct rq *rq, struct task_struct *p);
+	void (*put_prev_task) (struct rq *rq, struct task_struct *p, bool ctxsw);
 
 #ifdef CONFIG_SMP
 	int  (*select_task_rq)(struct task_struct *p, int sd_flag, int flags);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c058189..6803fd1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3164,7 +3164,7 @@ static void put_prev_task(struct rq *rq, struct task_struct *prev)
 {
 	if (prev->on_rq || rq->skip_clock_update < 0)
 		update_rq_clock(rq);
-	prev->sched_class->put_prev_task(rq, prev);
+	prev->sched_class->put_prev_task(rq, prev, true);
 }
 
 /*
@@ -3248,6 +3248,7 @@ need_resched:
 	if (unlikely(!rq->nr_running))
 		idle_balance(cpu, rq);
 
+
 	put_prev_task(rq, prev);
 	next = pick_next_task(rq);
 	clear_tsk_need_resched(prev);
@@ -3857,7 +3858,7 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
 	if (on_rq)
 		dequeue_task(rq, p, 0);
 	if (running)
-		p->sched_class->put_prev_task(rq, p);
+		p->sched_class->put_prev_task(rq, p, false);
 
 	if (rt_prio(prio))
 		p->sched_class = &rt_sched_class;
@@ -4208,7 +4209,7 @@ recheck:
 	if (on_rq)
 		dequeue_task(rq, p, 0);
 	if (running)
-		p->sched_class->put_prev_task(rq, p);
+		p->sched_class->put_prev_task(rq, p, false);
 
 	p->sched_reset_on_fork = reset_on_fork;
 
@@ -5197,7 +5198,7 @@ static void migrate_tasks(unsigned int dead_cpu)
 
 		next = pick_next_task(rq);
 		BUG_ON(!next);
-		next->sched_class->put_prev_task(rq, next);
+		next->sched_class->put_prev_task(rq, next, false);
 
 		/* Find suitable destination for @next, with force if needed. */
 		dest_cpu = select_fallback_rq(dead_cpu, next);
@@ -7318,7 +7319,7 @@ void sched_move_task(struct task_struct *tsk)
 	if (on_rq)
 		dequeue_task(rq, tsk, 0);
 	if (unlikely(running))
-		tsk->sched_class->put_prev_task(rq, tsk);
+		tsk->sched_class->put_prev_task(rq, tsk, false);
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	if (tsk->sched_class->task_move_group)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 940e6d1..1c8a04e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2996,7 +2996,7 @@ static struct task_struct *pick_next_task_fair(struct rq *rq)
 /*
  * Account for a descheduled task:
  */
-static void put_prev_task_fair(struct rq *rq, struct task_struct *prev)
+static void put_prev_task_fair(struct rq *rq, struct task_struct *prev, bool ctxsw)
 {
 	struct sched_entity *se = &prev->se;
 	struct cfs_rq *cfs_rq;
@@ -3004,6 +3004,8 @@ static void put_prev_task_fair(struct rq *rq, struct task_struct *prev)
 	for_each_sched_entity(se) {
 		cfs_rq = cfs_rq_of(se);
 		put_prev_entity(cfs_rq, se);
+		if (ctxsw)
+			cfs_rq->nr_switches++;
 	}
 }
 
diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c
index b44d604..08c3ad3 100644
--- a/kernel/sched/idle_task.c
+++ b/kernel/sched/idle_task.c
@@ -42,7 +42,7 @@ dequeue_task_idle(struct rq *rq, struct task_struct *p, int flags)
 	raw_spin_lock_irq(&rq->lock);
 }
 
-static void put_prev_task_idle(struct rq *rq, struct task_struct *prev)
+static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, bool ctxsw)
 {
 }
 
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index c5565c3..afe5096 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1387,8 +1387,10 @@ static struct task_struct *pick_next_task_rt(struct rq *rq)
 	return p;
 }
 
-static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
+static void put_prev_task_rt(struct rq *rq, struct task_struct *p, bool ctxsw)
 {
+	struct sched_rt_entity *rt_se = &p->rt;
+	struct rt_rq *rt_rq = rt_rq_of_se(rt_se);
 	update_curr_rt(rq);
 
 	/*
@@ -1397,6 +1399,8 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
 	 */
 	if (on_rt_rq(&p->rt) && p->rt.nr_cpus_allowed > 1)
 		enqueue_pushable_task(rq, p);
+	if (ctxsw)
+		rt_rq->rt_nr_switches++;
 }
 
 #ifdef CONFIG_SMP
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index ba9dccf..7548814 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -237,6 +237,7 @@ struct cfs_rq {
 	struct list_head leaf_cfs_rq_list;
 	struct task_group *tg;	/* group that "owns" this runqueue */
 
+	u64 nr_switches;
 #ifdef CONFIG_SMP
 	/*
 	 *   h_load = weight * f(tg)
@@ -306,6 +307,8 @@ struct rt_rq {
 	struct rq *rq;
 	struct list_head leaf_rt_rq_list;
 	struct task_group *tg;
+
+	u64 rt_nr_switches;
 #endif
 };
 
diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c
index 7b386e8..19ff22d 100644
--- a/kernel/sched/stop_task.c
+++ b/kernel/sched/stop_task.c
@@ -50,7 +50,7 @@ static void yield_task_stop(struct rq *rq)
 	BUG(); /* the stop task should never yield, its pointless. */
 }
 
-static void put_prev_task_stop(struct rq *rq, struct task_struct *prev)
+static void put_prev_task_stop(struct rq *rq, struct task_struct *prev, bool ctxsw)
 {
 }
 
-- 
1.7.10.2

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