Re: [PATCH sched_ext/for-7.4-fixes] sched_ext: Fix stale pick when a wakeup resets the donor during dispatch

[Andrea Righi <arighi@nvidia.com>]

Hi Tejun,

On Fri, Oct 30, 2026 at 04:23:38AM -1000, Tejun Heo wrote:
> With proxy execution, do_pick_task_scx() passes rq->donor to dispatch_one()
> as @prev. Unlike rq->curr, the donor can be moved to another CPU by a wakeup
> while dispatch_one() has the rq lock released:
> 
> 1. dispatch_one() releases the rq lock, e.g. to move a task from another
>    CPU.
> 2. Another CPU wakes the donor, which proxy execution migrated to this CPU.
>    proxy_needs_return() dequeues it and resets the donor to rq->curr, and
>    the wakeup enqueues it, possibly on another CPU.
> 3. dispatch_one() sees @prev queued and returns SCX_DSP_PREV.
> 4. do_pick_task_scx() returns @prev, and this CPU runs a task that is queued
>    on another rq.
> 
> The other verdicts are stale too. If the dispatch finds nothing before the
> wakeup enqueues @prev, SCX_DSP_NONE picks idle, and put_prev_task_scx()
> parks rq->curr on the local DSQ with nothing to reschedule the CPU, or trips
> its SCX_OPS_ENQ_LAST warning. SCX_DSP_LOCAL preempts rq->curr even when the
> dispatch would have kept it.
> 
> rq_modified_above() catches the reset only when rq->curr is in a class above
> ext, and the dl_server pick skips the check.
> 
> The verdict, whatever it is, describes the old donor. Restart the pick when
> @prev is no longer the donor, as dispatch_core_pick() already does whenever
> the dispatch released the rq lock. The retry runs with rq->curr as the
> donor, and proxy_needs_return() returns early for the current task, so the
> donor can't change again.
> 
> 7a3a6bfbd62a ("sched: Rework prev_balance() to avoid stale prev references")
> closed the same window in prev_balance() by reading rq->donor instead of
> taking @prev. That doesn't work here, as the dispatch never evaluated
> rq->curr, which may not even be a sched_ext task.
> 
> Verified with a synthetic reproducer.
> 
> Fixes: c6fe97c34a1a ("sched: Allow enabling proxy exec with sched_ext")
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---
>  kernel/sched/ext/ext.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
> index 15123f181b5b..3119a1938c6e 100644
> --- a/kernel/sched/ext/ext.c
> +++ b/kernel/sched/ext/ext.c
> @@ -3936,6 +3936,16 @@ static enum scx_dsp_verdict dispatch_pick(struct rq *rq, struct rq_flags *rf,
>  	verdict = dispatch_one(rq, prev);
>  	rq_repin_lock(rq, rf);
>  
> +	/*
> +	 * With proxy execution, @prev is the donor and need not be the running
> +	 * task. A wakeup while dispatch_one() has the rq lock released can
> +	 * reset the donor and move @prev to another CPU. The verdict was
> +	 * computed for @prev and doesn't apply once the donor changed. Restart
> +	 * the pick.
> +	 */
> +	if (unlikely(prev != rq->donor))
> +		return SCX_DSP_RETRY;
> +

I think Sashiko's first point is that this check comes too late to
protect accesses inside dispatch_one(). If the donor changes while
scx_process_sync_ecaps() drops the rq lock, the subsequent
prev->sched_class test checks the old donor, but update_curr_scx(rq)
operates on the new rq->donor, which might not be an EXT task.

Maybe we can detect the donor change before those accesses too?

And the second finding also looks valid, __pick_task_dl() sets
rq->dl_server even when the server pick returns RETRY_TASK, but
__pick_next_task() skips clearing it on restart. If the next pass picks
a non-server task, it inherits the stale server association.

Should we clear rq->dl_server on restart? (I haven't reproduced or
tested this).

Thanks,
-Andrea

>  	return verdict;
>  }
>  
> -- 
> 2.56.0
>