Re: [PATCH 0/4] sched/fair: Manage lag and run to parity with different slices

[Peter Zijlstra]

On Fri, Jun 13, 2025 at 04:05:10PM +0200, Vincent Guittot wrote:
> Vincent Guittot (3):
>   sched/fair: Use protect_slice() instead of direct comparison
>   sched/fair: Limit run to parity to the min slice of enqueued entities
>   sched/fair: Improve NO_RUN_TO_PARITY

Ah. I wrote these here patches and then totally forgot about them :/.
They take a different approach.

The approach I took was to move decision to stick with curr after pick,
instead of before it. That way we can evaluate the tree at the time of
preemption.


Subject: sched/fair: Fix requeue_delayed_entity()
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Fri Apr  4 11:23:00 CEST 2025

Since enqueue_task_fair() doesn't call update_curr() before calling
requeue_delayed_entity(), which means that update_entity_lag() uses a
slightly out-of-date avg_vruntime() -- which includes current.

Fixes: 54a58a787791 ("sched/fair: Implement DELAY_ZERO")
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/fair.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6893,6 +6893,8 @@ requeue_delayed_entity(struct sched_enti
 {
 	struct cfs_rq *cfs_rq = cfs_rq_of(se);
 
+	update_curr(cfs_rq);
+
 	/*
 	 * se->sched_delayed should imply: se->on_rq == 1.
 	 * Because a delayed entity is one that is still on
Subject: sched/eevdf: Re-arrange current protection in pick_eevdf()
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Fri Apr 4 10:15:15 CEST 2025

The way pick_eevdf() limits preemption is by explicitly picking
current if it is still eligible. It does this without consideration of
the best in-tree task.

Move current protection to after the tree selection such that a follow
up patch can change the conditions.

Should be an semantics no-op at this point.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/fair.c |   33 +++++++++++++++++++++++++--------
 1 file changed, 25 insertions(+), 8 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -903,6 +903,26 @@ static inline void cancel_protect_slice(
 		se->vlag = se->deadline + 1;
 }
 
+static inline bool __pick_eevdf_curr(struct cfs_rq *cfs_rq, struct sched_entity *best)
+{
+	struct sched_entity *curr = cfs_rq->curr;
+
+	/* only called when this is already checked */
+	WARN_ON_ONCE(!curr || !curr->on_rq);
+
+	/*
+	 * Strictly speaking we should allow current to run until its
+	 * deadline. However allow (wakeup) preemption once it is no longer
+	 * eligible.
+	 */
+	if (sched_feat(RUN_TO_PARITY) &&
+	    protect_slice(curr) &&
+	    entity_eligible(cfs_rq, curr))
+		return true;
+
+	return entity_before(curr, best);
+}
+
 /*
  * Earliest Eligible Virtual Deadline First
  *
@@ -929,18 +949,15 @@ static struct sched_entity *pick_eevdf(s
 	struct sched_entity *curr = cfs_rq->curr;
 	struct sched_entity *best = NULL;
 
+	if (curr && !curr->on_rq)
+		curr = NULL;
+
 	/*
 	 * We can safely skip eligibility check if there is only one entity
 	 * in this cfs_rq, saving some cycles.
 	 */
 	if (cfs_rq->nr_queued == 1)
-		return curr && curr->on_rq ? curr : se;
-
-	if (curr && (!curr->on_rq || !entity_eligible(cfs_rq, curr)))
-		curr = NULL;
-
-	if (sched_feat(RUN_TO_PARITY) && curr && protect_slice(curr))
-		return curr;
+		return curr ?: se;
 
 	/* Pick the leftmost entity if it's eligible */
 	if (se && entity_eligible(cfs_rq, se)) {
@@ -977,7 +994,7 @@ static struct sched_entity *pick_eevdf(s
 		node = node->rb_right;
 	}
 found:
-	if (!best || (curr && entity_before(curr, best)))
+	if (!best || (curr && __pick_eevdf_curr(cfs_rq, best)))
 		best = curr;
 
 	return best;
Subject: sched/eevdf: Fix RUN_TO_PARITY vs PREEMPT_SHORT
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Fri Apr 4 10:25:03 CEST 2025

Vincent noted that RUN_TO_PARITY can prevent preemption by a shorter
slice under some conditions.

Reported-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/fair.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -918,7 +918,7 @@ static inline bool __pick_eevdf_curr(str
 	if (sched_feat(RUN_TO_PARITY) &&
 	    protect_slice(curr) &&
 	    entity_eligible(cfs_rq, curr))
-		return true;
+		return !sched_feat(PREEMPT_SHORT) || curr->slice < best->slice;
 
 	return entity_before(curr, best);
 }
@@ -1195,7 +1195,7 @@ static inline bool did_preempt_short(str
 	if (!sched_feat(PREEMPT_SHORT))
 		return false;
 
-	if (curr->vlag == curr->deadline)
+	if (protect_slice(curr))
 		return false;
 
 	return !entity_eligible(cfs_rq, curr);