[PATCH v2] sched: fix incorrect wait time and wait count statistics

[Joonwoo Park]

At present scheduler resets task's wait start timestamp when the task
migrates to another rq.  This misleads scheduler itself into reporting
less wait time than actual by omitting time spent for waiting prior to
migration and also more wait count than actual by counting migration as
wait end event which can be seen by trace or /proc/<pid>/sched with
CONFIG_SCHEDSTATS=y.

Carry forward migrating task's wait time prior to migration and
don't count migration as a wait end event to fix such statistics error.

In order to determine whether task is migrating mark task->on_rq with
TASK_ON_RQ_MIGRATING while dequeuing and enqueuing due to migration.

To: Ingo Molnar <mingo@xxxxxxxxxx>
To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Joonwoo Park <joonwoop@xxxxxxxxxxxxxx>
---
Changes in v2: 
 * Set p->on_rq = TASK_ON_RQ_MIGRATING while doing migration dequeue/enqueue
   and check whether task's migrating with task_on_rq_migrating().

 kernel/sched/core.c |  4 ++--
 kernel/sched/fair.c | 17 ++++++++++++++---
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bcd214e..d9e4ad5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1069,8 +1069,8 @@ static struct rq *move_queued_task(struct rq *rq, struct task_struct *p, int new
 {
 	lockdep_assert_held(&rq->lock);
 
-	dequeue_task(rq, p, 0);
 	p->on_rq = TASK_ON_RQ_MIGRATING;
+	dequeue_task(rq, p, 0);
 	set_task_cpu(p, new_cpu);
 	raw_spin_unlock(&rq->lock);
 
@@ -1078,8 +1078,8 @@ static struct rq *move_queued_task(struct rq *rq, struct task_struct *p, int new
 
 	raw_spin_lock(&rq->lock);
 	BUG_ON(task_cpu(p) != new_cpu);
-	p->on_rq = TASK_ON_RQ_QUEUED;
 	enqueue_task(rq, p, 0);
+	p->on_rq = TASK_ON_RQ_QUEUED;
 	check_preempt_curr(rq, p, 0);
 
 	return rq;
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9a5e60f..7609576 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -740,7 +740,11 @@ static void update_curr_fair(struct rq *rq)
 static inline void
 update_stats_wait_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
-	schedstat_set(se->statistics.wait_start, rq_clock(rq_of(cfs_rq)));
+	schedstat_set(se->statistics.wait_start,
+		task_on_rq_migrating(task_of(se)) &&
+		likely(rq_clock(rq_of(cfs_rq)) > se->statistics.wait_start) ?
+		rq_clock(rq_of(cfs_rq)) - se->statistics.wait_start :
+		rq_clock(rq_of(cfs_rq)));
 }
 
 /*
@@ -759,6 +763,13 @@ static void update_stats_enqueue(struct cfs_rq *cfs_rq, struct sched_entity *se)
 static void
 update_stats_wait_end(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
+	if (task_on_rq_migrating(task_of(se))) {
+		schedstat_set(se->statistics.wait_start,
+			      rq_clock(rq_of(cfs_rq)) -
+			      se->statistics.wait_start);
+		return;
+	}
+
 	schedstat_set(se->statistics.wait_max, max(se->statistics.wait_max,
 			rq_clock(rq_of(cfs_rq)) - se->statistics.wait_start));
 	schedstat_set(se->statistics.wait_count, se->statistics.wait_count + 1);
@@ -5656,8 +5667,8 @@ static void detach_task(struct task_struct *p, struct lb_env *env)
 {
 	lockdep_assert_held(&env->src_rq->lock);
 
-	deactivate_task(env->src_rq, p, 0);
 	p->on_rq = TASK_ON_RQ_MIGRATING;
+	deactivate_task(env->src_rq, p, 0);
 	set_task_cpu(p, env->dst_cpu);
 }
 
@@ -5790,8 +5801,8 @@ static void attach_task(struct rq *rq, struct task_struct *p)
 	lockdep_assert_held(&rq->lock);
 
 	BUG_ON(task_rq(p) != rq);
-	p->on_rq = TASK_ON_RQ_QUEUED;
 	activate_task(rq, p, 0);
+	p->on_rq = TASK_ON_RQ_QUEUED;
 	check_preempt_curr(rq, p, 0);
 }
 
-- 
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