linux-kernel - Re: [PATCH 4/7] sched_ext: Add a DL server for sched

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Message-ID: <aXDLYj0_VnbVZkJE@gpd4>
Date: Wed, 21 Jan 2026 13:49:38 +0100
From: Andrea Righi <arighi@...dia.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>, 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>,
	Joel Fernandes <joelagnelf@...dia.com>, Tejun Heo <tj@...nel.org>,
	David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>, Shuah Khan <shuah@...nel.org>,
	sched-ext@...ts.linux.dev, bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Christian Loehle <christian.loehle@....com>
Subject: Re: [PATCH 4/7] sched_ext: Add a DL server for sched_ext tasks

On Wed, Jan 21, 2026 at 01:29:01PM +0100, Peter Zijlstra wrote:
> On Tue, Jan 20, 2026 at 10:50:35PM +0100, Andrea Righi wrote:
> 
> > @@ -1813,6 +1813,7 @@ void dl_server_stop(struct sched_dl_entity *dl_se)
> >  	hrtimer_try_to_cancel(&dl_se->dl_timer);
> >  	dl_se->dl_defer_armed = 0;
> >  	dl_se->dl_throttled = 0;
> > +	dl_se->dl_defer_running = 0;
> >  	dl_se->dl_defer_idle = 0;
> >  	dl_se->dl_server_active = 0;
> >  }
> 
> This should definitely not be in this patch. Why was this added? Were
> you trying to do the same as:
> 
>   ca1e8eede4fc ("sched/deadline: Fix server stopping with runnable tasks")
> 

The problem is that if remove this, RT can completely stall EXT tasks, also
with ca1e8eede4fc applied.

Example (with the rt_stall kselftest):

 # Runtime of EXT task (PID 2100) is 0.000000 seconds
 # Runtime of RT task (PID 2101) 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

Thanks,
-Andrea