[PATCH sched_ext/for-7.1-fixes] sched_ext: Don't warn on NULL cgrp_moving_from in scx_cgroup_move_task()

[Tejun Heo]

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.

Fixes: 819513666966 ("sched_ext: Add cgroup support")
Cc: stable@xxxxxxxxxxxxxxx # v6.12+
Reported-by: Matt Fleming <mfleming@xxxxxxxxxxxxxx>
Closes: https://lore.kernel.org/all/20260601124156.2205704-1-mfleming@xxxxxxxxxxxxxx/
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
 kernel/sched/ext.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 012ca8b..a1f7698 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -4293,11 +4293,13 @@ 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)));