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.

Fixes: 819513666966 ("sched_ext: Add cgroup support")
Cc: stable@vger.kernel.org # v6.12+
Reported-by: Matt Fleming <mfleming@cloudflare.com>
Closes: https://lore.kernel.org/all/20260601124156.2205704-1-mfleming@cloudflare.com/
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index c1762420cc35cd66df59b45a52174047701740b5..8e88a25bc602f4114cc600021dd8c921472f624c 100644 (file)
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -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)));