[PATCH] sched/stats: Fix run_delay over-count for migrated sched_delayed tasks

[albin_yang]

From: Wei Yang <albinwyang@xxxxxxxxxxx>

With DELAY_DEQUEUE, a blocked task stays on the runqueue with
se.sched_delayed set and its sched_info.last_queued is cleared, so the sleep
is not counted into run_delay.

When such a delayed (sleeping) task is migrated across CPUs via the plain
migration paths (move_queued_task / move_queued_task_locked, the latter used
by __migrate_swap_task), activate_task(dst, 0) calls enqueue_task() without
ENQUEUE_RESTORE, re-arming last_queued to the migration timestamp while the
task is still sleeping. The later real wakeup (ENQUEUE_DELAYED) tries to
re-arm last_queued at wakeup time but is suppressed because last_queued is
already non-zero, so sched_info_arrive() folds the whole sleep duration
between migration and wakeup into run_delay.

Fix by not re-arming last_queued for a sched_delayed task in
sched_info_enqueue(). The wakeup path clears sched_delayed before reaching
sched_info_enqueue(), so it still re-arms at the real wakeup time. Plain
runnable tasks and load-balance migrations are unaffected (sched_delayed
tasks are excluded from active load balancing).

Fixes: 152e11f6df29 ("sched/fair: Implement delayed dequeue")
Signed-off-by: Wei Yang <albinwyang@xxxxxxxxxxx>
---
 kernel/sched/stats.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/stats.h b/kernel/sched/stats.h
index ebe0a7765f98..dc626f99ffd9 100644
--- a/kernel/sched/stats.h
+++ b/kernel/sched/stats.h
@@ -290,7 +290,7 @@ static void sched_info_arrive(struct rq *rq, struct task_struct *t)
  */
 static inline void sched_info_enqueue(struct rq *rq, struct task_struct *t)
 {
-	if (!t->sched_info.last_queued)
+	if (!t->sched_info.last_queued && !t->se.sched_delayed)
 		t->sched_info.last_queued = rq_clock(rq);
 }
 
-- 
2.43.7