Re: [RFC PATCH] sched/deadline: Avoid dl_server boosting with expired deadline

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 07, 2025 at 02:29:04PM +0200, Gabriele Monaco wrote:
> Recent changes to the deadline server leave it running when the system
> is idle. If the system is idle for longer than the dl_server period and
> the first scheduling occurs after a fair task wakes up, the algorithm
> picks the server as the earliest deadline (in the past) and that boosts
> the fair task that just woke up while:
>  * the deadline is in the past
>  * the server consumed all its runtime (in background)
>  * there is no starvation (idle for about a period)
> 
> Prevent the server from boosting a task when the deadline is in the
> past. Instead, replenish a new period and start the server as deferred.
> 
> Fixes: 4ae8d9aa9f9d ("sched/deadline: Fix dl_server getting stuck")
> To: Juri Lelli <juri.lelli@redhat.com>
> Cc: Clark Williams <williams@redhat.com>
> Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
> ---
> 
> This behaviour was observed using the RV monitors in [1] and the patch
> was validated on an adapted version of the models. The models are not
> exhaustively validating the dl_server behaviour.
> 
> [1] - https://lore.kernel.org/lkml/20250919140954.104920-21-gmonaco@redhat.com
> 
>  kernel/sched/deadline.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 72c1f72463c7..b3e3d506a18d 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -2371,6 +2371,17 @@ static struct task_struct *__pick_task_dl(struct rq *rq)
>  			dl_server_stop(dl_se);
>  			goto again;
>  		}
> +		/*
> +		 * If the CPU was idle for long enough time and wakes up
> +		 * because of a fair task, the dl_server may run after its
> +		 * period elapsed. Replenish a new period as deferred, since we
> +		 * are clearly not handling starvation here.
> +		 */
> +		if (dl_time_before(dl_se->deadline, rq_clock(rq))) {
> +			dl_se->dl_defer_running = 0;
> +			replenish_dl_new_period(dl_se, rq);
> +			goto again;
> +		}
>  		rq->dl_server = dl_se;
>  	} else {
>  		p = dl_task_of(dl_se);
> 

I'm a bit confused, should not enqueue ensure deadline is in the future?
And if it doesn't shouldn't we fix the enqueue path somewhere?