linux-kernel - Re: [PATCH v2] sched/deadline: Reset dl

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Message-ID: <aXkJ8_zLVSuToLxe@gpd4>
Date: Tue, 27 Jan 2026 19:54:43 +0100
From: Andrea Righi <arighi@...dia.com>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	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>, Tejun Heo <tj@...nel.org>,
	Joel Fernandes <joelagnelf@...dia.com>,
	David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>,
	Daniel Hodges <hodgesd@...a.com>, sched-ext@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/deadline: Reset dl_server execution state on
 stop

On Tue, Jan 27, 2026 at 04:00:31PM +0000, Gabriele Monaco wrote:
> 2026-01-27T14:18:29Z Andrea Righi <arighi@...dia.com>:
> > I think my case should work also doing something like this (I'll run some
> > tests later to double check):
> >
> >     if (dl_se->runtime <= 0)
> >         dl_se->dl_defer_running = 0;
> >
> > In this way:
> > - short sleep + remaining runtime > 0
> >    - dl_defer_running stays set
> >    - restart can go A->D directly
> >    - no extra defer / zero-laxity penalty
> >
> > - stop with exhausted (or negative) runtime
> >    - dl_defer_running is cleared
> >    - restart must re-establish eligibility
> >    - deferral / timer is armed again
> >    - no stale "already running" server
> 
> Yeah that looks like the neatest to me.
> Fair tasks are a bit more penalised than now but won't be if they really sleep before consuming the runtime, which I think was the whole point of this logic.

Unfortunately checking only runtime <= 0 isn't enough for the sched_ext DL
server case:

 # Runtime of EXT task (PID 2025) is 0.000000 seconds
 # Runtime of RT task (PID 2026) is 4.990000 seconds
 # EXT task got 0.00% of total runtime
 not ok 2 FAIL: EXT task got less than 4.00% of runtime

With the unconditional reset the EXT task gets 5% of the bandwidth. I'll
add some debugging to figure out exactly what is happening.

-Andrea