linux-kernel - Re: [PATCH] sched/eevdf: Update se->vprot in reweight

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Message-ID: <20260123153854.GU166857@noisy.programming.kicks-ass.net>
Date: Fri, 23 Jan 2026 16:38:54 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: "wangtao (EQ)" <wangtao554@...wei.com>,
	K Prateek Nayak <kprateek.nayak@....com>, mingo@...hat.com,
	juri.lelli@...hat.com, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, tanghui20@...wei.com, zhangqiao22@...wei.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/eevdf: Update se->vprot in reweight_entity()

On Wed, Jan 21, 2026 at 06:00:15PM +0100, Vincent Guittot wrote:

> Why not use update_protect_slice() like when a new task with a shorter
> slice is added ?

That seems wrong too...

I was going over this, and should we not limit set_protect_slice() to
the first set_next_entity(). That is, AFAICT we'll re-set it on
sched_change -- which sounds wrong to me.

Anyway, I ended up with something like so (should probably be split in
two patches).

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index eca642295c4b..bab51da3d179 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3790,6 +3790,7 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
 			    unsigned long weight)
 {
 	bool curr = cfs_rq->curr == se;
+	u64 vprot = 0;
 
 	if (se->on_rq) {
 		/* commit outstanding execution time */
@@ -3797,6 +3798,9 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
 		update_entity_lag(cfs_rq, se);
 		se->deadline -= se->vruntime;
 		se->rel_deadline = 1;
+		if (curr && protect_slice(se))
+			vprot = se->vprot - se->vruntime;
+
 		cfs_rq->nr_queued--;
 		if (!curr)
 			__dequeue_entity(cfs_rq, se);
@@ -3812,6 +3816,9 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
 	if (se->rel_deadline)
 		se->deadline = div_s64(se->deadline * se->load.weight, weight);
 
+	if (vprot)
+		vprot = div_s64(vprot * se->load.weight, weight);
+
 	update_load_set(&se->load, weight);
 
 	do {
@@ -3823,6 +3830,8 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
 	enqueue_load_avg(cfs_rq, se);
 	if (se->on_rq) {
 		place_entity(cfs_rq, se, 0);
+		if (vprot)
+			se->vprot = se->vruntime + vprot;
 		update_load_add(&cfs_rq->load, se->load.weight);
 		if (!curr)
 			__enqueue_entity(cfs_rq, se);
@@ -5420,7 +5429,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 }
 
 static void
-set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
+set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, bool first)
 {
 	clear_buddies(cfs_rq, se);
 
@@ -5435,7 +5444,8 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
 		__dequeue_entity(cfs_rq, se);
 		update_load_avg(cfs_rq, se, UPDATE_TG);
 
-		set_protect_slice(cfs_rq, se);
+		if (first)
+			set_protect_slice(cfs_rq, se);
 	}
 
 	update_stats_curr_start(cfs_rq, se);
@@ -8958,13 +8968,13 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
 				pse = parent_entity(pse);
 			}
 			if (se_depth >= pse_depth) {
-				set_next_entity(cfs_rq_of(se), se);
+				set_next_entity(cfs_rq_of(se), se, true);
 				se = parent_entity(se);
 			}
 		}
 
 		put_prev_entity(cfs_rq, pse);
-		set_next_entity(cfs_rq, se);
+		set_next_entity(cfs_rq, se, true);
 
 		__set_next_task_fair(rq, p, true);
 	}
@@ -13578,7 +13588,7 @@ static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first)
 	for_each_sched_entity(se) {
 		struct cfs_rq *cfs_rq = cfs_rq_of(se);
 
-		set_next_entity(cfs_rq, se);
+		set_next_entity(cfs_rq, se, first);
 		/* ensure bandwidth has been allocated on our new cfs_rq */
 		account_cfs_rq_runtime(cfs_rq, 0);
 	}