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

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Message-ID: <aIHtfRB6UuVVbp2z@jlelli-thinkpadt14gen4.remote.csb>
Date: Thu, 24 Jul 2025 10:23:25 +0200
From: "juri.lelli@...hat.com" <juri.lelli@...hat.com>
To: Pierce Wen (溫彥翔) <Pierce.Wen@...iatek.com>
Cc: Kuyo Chang (張建文) <Kuyo.Chang@...iatek.com>,
	"bsegall@...gle.com" <bsegall@...gle.com>,
	"vschneid@...hat.com" <vschneid@...hat.com>,
	"dietmar.eggemann@....com" <dietmar.eggemann@....com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"mgorman@...e.de" <mgorman@...e.de>,
	"jstultz@...gle.com" <jstultz@...gle.com>,
	"matthias.bgg@...il.com" <matthias.bgg@...il.com>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mediatek@...ts.infradead.org" <linux-mediatek@...ts.infradead.org>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
Subject: Re: [RFC PATCH 1/1] sched/deadline: Fix RT task potential starvation
 when expiry time passed

On 23/07/25 22:22, Pierce Wen (溫彥翔) wrote:

...

> Hi all,

Hello!

> I wanted to follow up on the discussion regarding the potential RT task
> starvation issue and check if there have been any further updates or
> feedback.
> 
> To recap and provide some additional context:
> 
> 1. As discussed in the thread (see
> https://lore.kernel.org/all/CANDhNCqYCpdhYS9afdKeY34Bmw8MXyqKWCSTxOZNLTjYrUaVXg@mail.gmail.com/
> ), it has been demonstrated that the use of a scaled timer can indeed
> induce RT starvation under certain conditions.

This first case has been handled by fc975cfb36393 ("sched/deadline: Fix
dl_server runtime calculation formula") on tip/master.

> 2. Furthermore, since the delta_exec time calculation relies on the
> clock_task member of struct rq, which is affected by IRQ time on the
> runqueue, there is a risk that if IRQ time becomes excessively long in
> some corner cases, it could also lead to RT starvation.
> 
> 3. Based on these observations, we strongly recommend adopting a
> recovery patch to address these critical scenarios and prevent RT task
> starvation, especially in cases where the current logic may not be
> sufficient.

As I was saying, I am not against the patch proposed to address the
starvation issue discussed in this thread. Maybe the patch can be
reposted with the addition of a comment on the code path related to
dl-server explaining why the special case is needed?

Thanks,
Juri