Re: [RFC][PATCH] sched/deadline: Fix dl_server getting stuck, allowing cpu starvation

[Juri Lelli <juri.lelli@redhat.com>]

On 16/09/25 13:01, Peter Zijlstra wrote:
> On Tue, Sep 16, 2025 at 10:51:34AM +0200, Juri Lelli wrote:
> 
> > > @@ -1173,7 +1171,7 @@ static enum hrtimer_restart dl_server_timer(struct hrtimer *timer, struct sched_
> > >  
> > >  		if (!dl_se->server_has_tasks(dl_se)) {
> > >  			replenish_dl_entity(dl_se);
> > > -			dl_server_stopped(dl_se);
> > > +			dl_server_stop(dl_se);
> > >  			return HRTIMER_NORESTART;
> > >  		}
> > 
> > It looks OK for a quick testing I've done. Also, it seems to make sense
> > to me. The defer timer has fired (we are executing the callback). If the
> > server hasn't got tasks to serve we can just stop it (clearing the
> > flags) and wait for the next enqueue of fair to start it again still in
> > defer mode. hrtimer_try_to_cancel() is redundant (but harmless),
> > dequeue_dl_entity() I believe we need to call to deal with
> > task_non_contending().
> > 
> > Peter, what do you think?
> 
> Well, the problem was that we were starting/stopping the thing too
> often, and the general idea of that commit:
> 
>   cccb45d7c4295 ("sched/deadline: Less agressive dl_server handling")
> 
> was to not stop the server, unless it's not seen fair tasks for a whole
> period.
> 
> Now, the case John trips seems to be that there were tasks, we ran tasks
> until budget exhausted, dequeued the server and did start_dl_timer().
> 
> Then the bandwidth timer fires at a point where there are no more fair
> tasks, replenish_dl_entity() gets called, which *should* set the
> 0-laxity timer, but doesn't -- because !server_has_tasks() -- and then
> nothing.
> 
> So perhaps we should do something like the below. Simply continue
> as normal, until we do a whole cycle without having seen a task.
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 5b64bc621993..269ca2eb5ba9 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -875,7 +875,7 @@ static void replenish_dl_entity(struct sched_dl_entity *dl_se)
>  	 */
>  	if (dl_se->dl_defer && !dl_se->dl_defer_running &&
>  	    dl_time_before(rq_clock(dl_se->rq), dl_se->deadline - dl_se->runtime)) {
> -		if (!is_dl_boosted(dl_se) && dl_se->server_has_tasks(dl_se)) {
> +		if (!is_dl_boosted(dl_se)) {
>  
>  			/*
>  			 * Set dl_se->dl_defer_armed and dl_throttled variables to
> @@ -1171,12 +1171,6 @@ static enum hrtimer_restart dl_server_timer(struct hrtimer *timer, struct sched_
>  		if (!dl_se->dl_runtime)
>  			return HRTIMER_NORESTART;
>  
> -		if (!dl_se->server_has_tasks(dl_se)) {
> -			replenish_dl_entity(dl_se);
> -			dl_server_stopped(dl_se);
> -			return HRTIMER_NORESTART;
> -		}
> -
>  		if (dl_se->dl_defer_armed) {
>  			/*
>  			 * First check if the server could consume runtime in background.
> 
> 
> Notably, this removes all ->server_has_tasks() users, so if this works
> and is correct, we can completely remove that callback and simplify
> more.
> 
> Hmm?

But then what stops the server when the 0-laxity (defer) timer fires
again a period down the line?