[PATCH v2] sched/fair: sanitize vruntime of entity being migrated

[Zhang Qiao]

Commit 829c1651e9c4 ("sched/fair: sanitize vruntime of
entity being placed") fix an overflowing bug, but ignore
a case that se->exec_start is reset after a migration.

For fixing this case, we reset the vruntime of a long
sleeping task in migrate_task_rq_fair().

Fixes: 829c1651e9c4 ("sched/fair: sanitize vruntime of entity being placed")
Suggested-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Zhang Qiao <zhangqiao22@xxxxxxxxxx>
---

v1 -> v2:
- fix some typos and update comments
- reformat the patch

---
 kernel/sched/fair.c | 76 ++++++++++++++++++++++++++++++++-------------
 1 file changed, 55 insertions(+), 21 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7a1b1f855b96..74c9918ffe76 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4648,11 +4648,45 @@ static void check_spread(struct cfs_rq *cfs_rq, struct sched_entity *se)
 #endif
 }
 
+static inline bool entity_is_long_sleep(struct sched_entity *se)
+{
+	struct cfs_rq *cfs_rq;
+	u64 sleep_time;
+
+	if (se->exec_start == 0)
+		return false;
+
+	cfs_rq = cfs_rq_of(se);
+	sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start;
+	if ((s64)sleep_time > 60LL * NSEC_PER_SEC)
+		return true;
+
+	return false;
+}
+
+static inline u64 sched_sleeper_credit(struct sched_entity *se)
+{
+	unsigned long thresh;
+
+	if (se_is_idle(se))
+		thresh = sysctl_sched_min_granularity;
+	else
+		thresh = sysctl_sched_latency;
+
+	/*
+	 * Halve their sleep time's effect, to allow
+	 * for a gentler effect of sleepers:
+	 */
+	if (sched_feat(GENTLE_FAIR_SLEEPERS))
+		thresh >>= 1;
+
+	return thresh;
+}
+
 static void
 place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
 {
 	u64 vruntime = cfs_rq->min_vruntime;
-	u64 sleep_time;
 
 	/*
 	 * The 'current' period is already promised to the current tasks,
@@ -4664,23 +4698,8 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
 		vruntime += sched_vslice(cfs_rq, se);
 
 	/* sleeps up to a single latency don't count. */
-	if (!initial) {
-		unsigned long thresh;
-
-		if (se_is_idle(se))
-			thresh = sysctl_sched_min_granularity;
-		else
-			thresh = sysctl_sched_latency;
-
-		/*
-		 * Halve their sleep time's effect, to allow
-		 * for a gentler effect of sleepers:
-		 */
-		if (sched_feat(GENTLE_FAIR_SLEEPERS))
-			thresh >>= 1;
-
-		vruntime -= thresh;
-	}
+	if (!initial)
+		vruntime -= sched_sleeper_credit(se);
 
 	/*
 	 * Pull vruntime of the entity being placed to the base level of
@@ -4689,8 +4708,7 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
 	 * the base as it may be too far off and the comparison may get
 	 * inversed due to s64 overflow.
 	 */
-	sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start;
-	if ((s64)sleep_time > 60LL * NSEC_PER_SEC)
+	if (entity_is_long_sleep(se))
 		se->vruntime = vruntime;
 	else
 		se->vruntime = max_vruntime(se->vruntime, vruntime);
@@ -7635,7 +7653,23 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
 	if (READ_ONCE(p->__state) == TASK_WAKING) {
 		struct cfs_rq *cfs_rq = cfs_rq_of(se);
 
-		se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
+		/*
+		 * We determine whether a task sleeps for long by checking
+		 * se->exec_start, and if it is, we sanitize its vruntime at
+		 * place_entity(). However, after a migration, this detection
+		 * method fails due to se->exec_start being reset.
+		 *
+		 * For fixing this case, we add the same check here. For a task
+		 * which has slept for a long time, its vruntime should be reset
+		 * to cfs_rq->min_vruntime with a sleep credit. Because waking
+		 * task's vruntime will be added to cfs_rq->min_vruntime when
+		 * enqueue, we only need to reset the se->vruntime of waking task
+		 * to a credit here.
+		 */
+		if (entity_is_long_sleep(se))
+			se->vruntime = -sched_sleeper_credit(se);
+		else
+			se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
 	}
 
 	if (!task_on_rq_migrating(p)) {
-- 
2.17.1