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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 > Closes: https://lore.kernel.org/all/20260601124156.2205704-1-mfleming@cloudflare.com/ > Signed-off-by: Tejun Heo Makes sense to me. Reviewed-by: Andrea Righi Thanks, -Andrea > --- > 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)));