linux-kernel - Re: [RFC PATCH 1/1] sched/deadline: Fix RT task potential starvation when expiry time passed

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Message-ID: <aFQM8TdZIIvvGv8T@jlelli-thinkpadt14gen4.remote.csb>
Date: Thu, 19 Jun 2025 15:13:21 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Kuyo Chang <kuyo.chang@...iatek.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	jstultz <jstultz@...gle.com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-mediatek@...ts.infradead.org
Subject: Re: [RFC PATCH 1/1] sched/deadline: Fix RT task potential starvation
 when expiry time passed

On 18/06/25 22:20, Kuyo Chang wrote:

...

> When dl_defer_running = 1 and the running time has been exhausted, 
> it means that the dl_server should stop at this point.
> However, if start_dl_timer() returns a failure, it indicates that the
> actual time spent consuming the running time was unexpectedly long. 
>  
> At this point, there are two options:
> [as-is] 1. re-enqueuing the dl entity with ENQUEUE_REPLENISH will clear
> the throttled flag 
> and re-enqueue the dl entity to keep the fair_server running. 
> enqueue_dl_entity(dl_se, ENQUEUE_REPLENISH);
> => replenish_dl_entity
>   => replenish_dl_new_period(dl_se, rq);
>   => dl_se->dl_yielded = 0;
>   => dl_se->dl_throttled = 0;
> => __enqueue_dl_entity(dl_se);
> 
> [to-be] 2. To avoid RT latency, the fair_server should remain throttled
> while replenishing the dl_se. 
> Once replenishing is complete, we can ensure that a timer is
> successfully started. 
> When the timer is triggered, the throttled state will be cleared,
> ensuring that RT tasks can execute during this interval.
>  
> It is a policy decision for dealing with the case of failure in
> start_dl_timer().
> The second approach is better for real-time (RT) latency in my opinion,
> as RT tasks must be prioritized.

OK, I think I see your points, but I am still not sure I fully
understand the link with the issue you describe in the changelog - the
relation with "DL replenish lagged too much", that is.

Could you please expand on the details of the situation that is opening
up for the issue your patch is addressing? Do you know why we hit the
corner case that causes the warning in the first place?

I would like to understand exactly what we are trying to fix before
deciding how to fix it, sorry if I am being dense. :-)

Thanks,
Juri