FAILED: patch "[PATCH] sched_ext: Don't warn on NULL cgrp_moving_from in" failed to apply to 6.18-stable tree

The patch below does not apply to the 6.18-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-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 02e545c4297a26dbbc41df81b831e7f605bcd306
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026061553-elderly-speech-3446@gregkh' --subject-prefix 'PATCH 6.18.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

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

From 02e545c4297a26dbbc41df81b831e7f605bcd306 Mon Sep 17 00:00:00 2001
From: Tejun Heo <[email protected]>
Date: Mon, 1 Jun 2026 09:22:37 -1000
Subject: [PATCH] sched_ext: Don't warn on NULL cgrp_moving_from in
scx_cgroup_move_task()

A WARN fires when systemd's user manager writes "+cpu +memory +pids" to its own subtree_control while a sched_ext scheduler is loaded:

  WARNING: at kernel/sched/ext.c:3227 scx_cgroup_move_task+0xa8/0xb0
   scx_cgroup_move_task+0xa8/0xb0
   sched_move_task+0x134/0x290
   cpu_cgroup_attach+0x39/0x70
   cgroup_migrate_execute+0x37d/0x450
   cgroup_update_dfl_csses+0x1e3/0x270
   cgroup_subtree_control_write+0x3e7/0x440

scx_cgroup_can_attach() arms cgrp_moving_from only when a task's cpu cgroup changes. It can still be NULL when scx_cgroup_move_task() runs, through this sequence:

  Step                               Result
  ---------------------------------  ----------------------------------
  1. cpu enabled on cgroup G         cpu css = A
  2. cpu toggled off then on for G   A killed, B created (same cgroup)
  3. an exiting task keeps A alive   migration skips it, A now stale
  4. +memory migrates G              stale A vs current B pulls cpu in
  5. cpu attach runs for all tasks   hits a live, cpu-unchanged task
  6. scx_cgroup_move_task() on it    cgrp_moving_from NULL -> WARN

The mismatch is that scx_cgroup_can_attach() keys on cgroup identity while migration drives the move on css identity, so a NULL cgrp_moving_from here is a legitimate css-only migration, not a missing prep.

The call is already gated on cgrp_moving_from, so just drop the warning. ops.cgroup_prep_move() and ops.cgroup_move() stay paired.

index c1762420cc35..8e88a25bc602 100644
@@ -4402,11 +4402,13 @@ void scx_cgroup_move_task(struct task_struct *p)
 		return;
 
 	/*
-	 * @p must have ops.cgroup_prep_move() called on it and thus
-	 * cgrp_moving_from set.
+	 * scx_cgroup_can_attach() sets cgrp_moving_from only when the task's
+	 * cgroup changes. Migration keys off css rather than cgroup identity,
+	 * so it can hand an unchanged-cgroup task here with cgrp_moving_from
+	 * NULL. Nothing to report to the BPF scheduler then, so skip it and
+	 * keep prep_move and move paired.
 	 */
-	if (SCX_HAS_OP(sch, cgroup_move) &&
-	    !WARN_ON_ONCE(!p->scx.cgrp_moving_from))
+	if (SCX_HAS_OP(sch, cgroup_move) && p->scx.cgrp_moving_from)
 		SCX_CALL_OP_TASK(sch, cgroup_move, task_rq(p),
 				 p, p->scx.cgrp_moving_from,
 				 tg_cgrp(task_group(p)));

From: Tejun Heo <[email protected]>

commit 02e545c4297a26dbbc41df81b831e7f605bcd306 upstream.

A WARN fires when systemd's user manager writes "+cpu +memory +pids" to its own subtree_control while a sched_ext scheduler is loaded:

  WARNING: at kernel/sched/ext.c:3227 scx_cgroup_move_task+0xa8/0xb0
   scx_cgroup_move_task+0xa8/0xb0
   sched_move_task+0x134/0x290
   cpu_cgroup_attach+0x39/0x70
   cgroup_migrate_execute+0x37d/0x450
   cgroup_update_dfl_csses+0x1e3/0x270
   cgroup_subtree_control_write+0x3e7/0x440

scx_cgroup_can_attach() arms cgrp_moving_from only when a task's cpu cgroup changes. It can still be NULL when scx_cgroup_move_task() runs, through this sequence:

  Step                               Result
  ---------------------------------  ----------------------------------
  1. cpu enabled on cgroup G         cpu css = A
  2. cpu toggled off then on for G   A killed, B created (same cgroup)
  3. an exiting task keeps A alive   migration skips it, A now stale
  4. +memory migrates G              stale A vs current B pulls cpu in
  5. cpu attach runs for all tasks   hits a live, cpu-unchanged task
  6. scx_cgroup_move_task() on it    cgrp_moving_from NULL -> WARN

The mismatch is that scx_cgroup_can_attach() keys on cgroup identity while migration drives the move on css identity, so a NULL cgrp_moving_from here is a legitimate css-only migration, not a missing prep.

The call is already gated on cgrp_moving_from, so just drop the warning. ops.cgroup_prep_move() and ops.cgroup_move() stay paired.

[ mfleming: keep the 6.18.y SCX_KF_REST argument in the
SCX_CALL_OP_TASK() call. ]

---
 kernel/sched/ext.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

index 7b750bf42698..d8280f874433 100644
@@ -3221,11 +3221,13 @@ void scx_cgroup_move_task(struct task_struct *p)
 		return;
 
 	/*
-	 * @p must have ops.cgroup_prep_move() called on it and thus
-	 * cgrp_moving_from set.
+	 * scx_cgroup_can_attach() sets cgrp_moving_from only when the task's
+	 * cgroup changes. Migration keys off css rather than cgroup identity,
+	 * so it can hand an unchanged-cgroup task here with cgrp_moving_from
+	 * NULL. Nothing to report to the BPF scheduler then, so skip it and
+	 * keep prep_move and move paired.
 	 */
-	if (SCX_HAS_OP(sch, cgroup_move) &&
-	    !WARN_ON_ONCE(!p->scx.cgrp_moving_from))
+	if (SCX_HAS_OP(sch, cgroup_move) && p->scx.cgrp_moving_from)
 		SCX_CALL_OP_TASK(sch, SCX_KF_REST, cgroup_move, task_rq(p),
 				 p, p->scx.cgrp_moving_from,
 				 tg_cgrp(task_group(p)));

2.43.0

Queued for 6.18.y, thanks.

--
Thanks,
Sasha