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

[Peter Zijlstra]

On Tue, Mar 14, 2023 at 08:41:30AM +0100, Vincent Guittot wrote:

> I'm going to use something a bit different from your proposal below by
> merging initial and flag
> static void place_entity(struct cfs_rq *cfs_rq, struct sched_entity
> *se, int flags)
> 
> with flags:
> 0 for initial placement
> ENQUEUE_WAKEUP for wakeup
> ENQUEUE_MIGRATED for migrated task

So when a task is not running for a long time (our case at hand), then
there's two cases:

 - it wakes up locally and place_entity() gets to reset vruntime;
 - it wakes up remotely and migrate_task_rq_fair() can reset vruntime.

So if we can rely on ENQUEUE_MIGRATED to differentiate between these
cases, when wouldn't something like this work?

---

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7a1b1f855b96..a0d00b6a8bc6 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4648,11 +4648,31 @@ static void check_spread(struct cfs_rq *cfs_rq, struct sched_entity *se)
 #endif
 }
 
+static bool reset_vruntime(struct cfs_rq *cfs_rq, struct sched_entity *se)
+{
+	const s64 limit = 60LL * NSEC_PER_SEC;
+	s64 sleep_time;
+
+	/*
+	 * Pull vruntime of the entity being placed to the base level of
+	 * cfs_rq, to prevent boosting it if placed backwards.  If the entity
+	 * slept for a long time, don't even try to compare its vruntime with
+	 * 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 (unlikely(sleep_time > limit)) {
+		se->vruntime = cfs_rq->min_vruntime - calc_delta_fair(limit, se);
+		return true;
+	}
+
+	return false;
+}
+
 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,
@@ -4682,18 +4702,8 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
 		vruntime -= thresh;
 	}
 
-	/*
-	 * Pull vruntime of the entity being placed to the base level of
-	 * cfs_rq, to prevent boosting it if placed backwards.  If the entity
-	 * slept for a long time, don't even try to compare its vruntime with
-	 * 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)
-		se->vruntime = vruntime;
-	else
-		se->vruntime = max_vruntime(se->vruntime, vruntime);
+	/* ensure we don't gain time by being placed backwards */
+	se->vruntime = max_vruntime(se->vruntime, vruntime);
 }
 
 static void check_enqueue_throttle(struct cfs_rq *cfs_rq);
@@ -4768,8 +4778,11 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 	update_cfs_group(se);
 	account_entity_enqueue(cfs_rq, se);
 
-	if (flags & ENQUEUE_WAKEUP)
+	if (flags & ENQUEUE_WAKEUP) {
+		if (!(flags & ENQUEUE_MIGRATED))
+			reset_vruntime(cfs_rq, se);
 		place_entity(cfs_rq, se, 0);
+	}
 
 	check_schedstat_required();
 	update_stats_enqueue_fair(cfs_rq, se, flags);
@@ -7625,6 +7638,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
 static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
 {
 	struct sched_entity *se = &p->se;
+	struct cfs_rq *cfs_rq = cfs_rq_of(se);
 
 	/*
 	 * As blocked tasks retain absolute vruntime the migration needs to
@@ -7632,11 +7646,8 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
 	 * min_vruntime -- the latter is done by enqueue_entity() when placing
 	 * the task on the new runqueue.
 	 */
-	if (READ_ONCE(p->__state) == TASK_WAKING) {
-		struct cfs_rq *cfs_rq = cfs_rq_of(se);
-
+	if (READ_ONCE(p->__state) == TASK_WAKING || reset_vruntime(cfs_rq, se))
 		se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
-	}
 
 	if (!task_on_rq_migrating(p)) {
 		remove_entity_load_avg(se);