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

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Message-ID: <aMvEsIkSv21kksrL@jlelli-thinkpadt14gen4.remote.csb>
Date: Thu, 18 Sep 2025 10:37:04 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: John Stultz <jstultz@...gle.com>, LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Valentin Schneider <vschneid@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Xuewen Yan <xuewen.yan94@...il.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Qais Yousef <qyousef@...alina.io>,
	Joel Fernandes <joelagnelf@...dia.com>,
	kuyo chang <kuyo.chang@...iatek.com>, hupu <hupu.gm@...il.com>,
	kernel-team@...roid.com
Subject: Re: [RFC][PATCH] sched/deadline: Fix dl_server getting stuck,
 allowing cpu starvation

On 17/09/25 19:30, Peter Zijlstra wrote:
> On Wed, Sep 17, 2025 at 03:56:20PM +0200, Juri Lelli wrote:
> 
> > > + * By stopping at this point the dl_server retains bandwidth, which, if a new
> > > + * task wakes up imminently (starting the server again), can be used --
> > > + * subject to CBS wakeup rules -- without having to wait for the next period.
> > 
> > In both cases we still defer until either the new period or the current
> > 0-laxity, right?
> > 
> > The stop cleans all the flags, so subsequent start calls
> > enqueue(ENQUEUE_WAKEUP) -> update_dl_entity() which sets dl_throttled
> > and dl_defer_armed in both cases and then we start_dl_timer (defer
> > timer) after it (without enqueueing right away).
> > 
> > Or maybe I am still a bit lost. :)
> 
> The way I read it earlier today:
> 
>   dl_server_start()
>     enqueue_dl_entity(WAKEUP)
>       if (WAKEUP)
> 	task_contending();
> 	update_dl_entity()
> 	  dl_entity_overflows() := true
> 	  update_dl_revised_wakeup();
> 
> In that case, it is possible to continue running with a slight
> adjustment to the runtime (it gets scaled back to account for 'lost'
> time or somesuch IIRC).
> 

Hummm, but this is for !implicit (dl_deadline != dl_period) tasks, is
it? And dl-servers are implicit.