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

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Message-ID: <20250917173038.GA973992@noisy.programming.kicks-ass.net>
Date: Wed, 17 Sep 2025 19:30:38 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Juri Lelli <juri.lelli@...hat.com>
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 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).