Re: [PATCH sched_ext/for-7.3-fixes] sched_ext: Unlink pending local reenqueues before freeing scheduler

[Tejun Heo]

Hello,

On Wed, Sep 16, 2026 at 04:58:07PM +0200, Andrea Righi wrote:
> A scheduler teardown enters bypass before the RCU grace period, which
> prevents new requests. However, this does not guarantee that an
> already-linked request has been consumed. An RCU grace period waits for
> active readers and it does not flush a pending rq deferred request.

Every link is followed by a scheduled run_deferred(), so a request that is
still linked when the scheduler is freed means that schedule was lost. The
WARN is doing its job there. Unlinking would hide the next one.

The lost schedule comes from move_remote_task_to_local_dsq(). It stashes the
mover's enq_flags for the destination enqueue and, since 57ccf5ccdc56
("sched_ext: Fix enqueue_task_scx() truncation of upper enqueue flags"),
those flags decide SCX_RQ_IN_WAKEUP. An IMMED insert into a busy remote CPU
requests a local reenqueue while the flag is set, schedule_deferred_locked()
leaves it to task_woken_scx(), and nothing calls that after activate_task().
Fix posted here:

  https://lore.kernel.org/all/20260916215713.2701551-1-tj@xxxxxxxxxx/

Thanks.

-- 
tejun