linux-kernel - Re: [PATCH 06/14] sched_ext: Add a DL server for sched

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Message-ID: <aPZCmZF_K-vJOYKp@jlelli-thinkpadt14gen4.remote.csb>
Date: Mon, 20 Oct 2025 16:09:29 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Andrea Righi <arighi@...dia.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	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,
	Luigi De Matteis <ldematteis123@...il.com>
Subject: Re: [PATCH 06/14] sched_ext: Add a DL server for sched_ext tasks

On 20/10/25 15:50, Andrea Righi wrote:
> Hi Juri,
> 
> On Mon, Oct 20, 2025 at 01:58:50PM +0200, Juri Lelli wrote:
> > Hi!
> > 
> > On 17/10/25 11:25, Andrea Righi wrote:

...

> > > @@ -1487,6 +1499,11 @@ static bool dequeue_task_scx(struct rq *rq, struct task_struct *p, int deq_flags
> > >  	sub_nr_running(rq, 1);
> > >  
> > >  	dispatch_dequeue(rq, p);
> > > +
> > > +	/* Stop the server if this was the last task */
> > > +	if (rq->scx.nr_running == 0)
> > > +		dl_server_stop(&rq->ext_server);
> > > +
> > 
> > Do we want to use the delayed stop behavior for scx-server as we have
> > for fair-server? Wonder if it's a matter of removing this explicit stop
> > and wait for a full period to elapse as we do for fair. It should reduce
> > timer reprogramming overhead for scx as well.
> 
> So, IIUC we could just remove this explicit dl_server_stop() and the server
> would naturally stop at the end of its current deadline period, if there
> are still no runnable tasks, right?

Right, that is what I'd expect. But this part tricked me several times
already, so I am not 100% certain (Peter please keep me honest :).

> In that case it's worth a try.