FAILED: patch "[PATCH] sched/fair: Adhere to place_entity() constraints" failed to apply to 6.12-stable tree

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x c70fc32f44431bb30f9025ce753ba8be25acbba3
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026031207-sessions-crablike-5f4d@gregkh' --subject-prefix 'PATCH 6.12.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From c70fc32f44431bb30f9025ce753ba8be25acbba3 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <[email protected]>
Date: Tue, 28 Jan 2025 15:39:49 +0100
Subject: [PATCH] sched/fair: Adhere to place_entity() constraints

Mike reports that commit 6d71a9c61604 ("sched/fair: Fix EEVDF entity placement bug causing scheduling lag") relies on commit 4423af84b297 ("sched/fair: optimize the PLACE_LAG when se->vlag is zero") to not trip a WARN in place_entity().

What happens is that the lag of the very last entity is 0 per definition -- the average of one element matches the value of that element. Therefore place_entity() will match the condition skipping the lag adjustment:

  if (sched_feat(PLACE_LAG) && cfs_rq->nr_queued && se->vlag) {

Without the 'se->vlag' condition -- it will attempt to adjust the zero lag even though we're inserting into an empty tree.

Notably, we should have failed the 'cfs_rq->nr_queued' condition, but don't because they didn't get updated.

Additionally, move update_load_add() after placement() as is consistent with other place_entity() users -- this change is non-functional, place_entity() does not use cfs_rq->load.

index 5e1bd9e8464c..eb5a2572b4f8 100644
@@ -3795,6 +3795,7 @@ 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;
+		cfs_rq->nr_queued--;
 		if (!curr)
 			__dequeue_entity(cfs_rq, se);
 		update_load_sub(&cfs_rq->load, se->load.weight);

@@ -3821,10 +3822,11 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
 
 	enqueue_load_avg(cfs_rq, se);
 	if (se->on_rq) {
-		update_load_add(&cfs_rq->load, se->load.weight);
 		place_entity(cfs_rq, se, 0);
+		update_load_add(&cfs_rq->load, se->load.weight);
 		if (!curr)
 			__enqueue_entity(cfs_rq, se);
+		cfs_rq->nr_queued++;
 
 		/*
 		 * The entity's vruntime has been adjusted, so let's check