linux-kernel - Re: [PATCH 06/11] sched_ext: Add a DL server for sched

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Message-ID: <aQzXv0iW3A1W2yow@gpd4>
Date: Thu, 6 Nov 2025 18:15:43 +0100
From: Andrea Righi <arighi@...dia.com>
To: Juri Lelli <juri.lelli@...hat.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>, Tejun Heo <tj@...nel.org>,
	David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>, Shuah Khan <shuah@...nel.org>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Christian Loehle <christian.loehle@....com>,
	Emil Tsalapatis <emil@...alapatis.com>,
	Luigi De Matteis <ldematteis123@...il.com>,
	sched-ext@...ts.linux.dev, bpf@...r.kernel.org,
	linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/11] sched_ext: Add a DL server for sched_ext tasks

On Thu, Nov 06, 2025 at 11:59:56AM +0100, Juri Lelli wrote:
> Hi,
> 
> On 29/10/25 20:08, Andrea Righi wrote:
> > sched_ext currently suffers starvation due to RT. The same workload when
> > converted to EXT can get zero runtime if RT is 100% running, causing EXT
> > processes to stall. Fix it by adding a DL server for EXT.
> > 
> > A kselftest is also provided later to verify:
> > 
> >  # ./runner -t rt_stall
> >  ===== START =====
> >  TEST: rt_stall
> >  DESCRIPTION: Verify that RT tasks cannot stall SCHED_EXT tasks
> >  OUTPUT:
> >  # Runtime of EXT task (PID 23338) is 0.250000 seconds
> >  # Runtime of RT task (PID 23339) is 4.750000 seconds
> >  # EXT task got 5.00% of total runtime
> >  ok 1 PASS: EXT task got more than 4.00% of runtime
> >  =====  END  =====
> > 
> > v3: - clarify that fair is not the only dl_server (Juri Lelli)
> >     - remove explicit stop to reduce timer reprogramming overhead
> >       (Juri Lelli)
> >     - do not restart pick_task() when it's invoked by the dl_server
> >       (Tejun Heo)
> >     - depend on CONFIG_SCHED_CLASS_EXT (Andrea Righi)
> > v2: - drop ->balance() now that pick_task() has an rf argument
> >       (Andrea Righi)
> > 
> > Cc: Luigi De Matteis <ldematteis123@...il.com>
> > Co-developed-by: Joel Fernandes <joelagnelf@...dia.com>
> > Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
> > Signed-off-by: Andrea Righi <arighi@...dia.com>
> > ---
> 
> ...
> 
> > @@ -1409,6 +1412,15 @@ static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int enq_flags
> >  	if (enq_flags & SCX_ENQ_WAKEUP)
> >  		touch_core_sched(rq, p);
> >  
> > +	if (rq->scx.nr_running == 1) {
> > +		/* Account for idle runtime */
> > +		if (!rq->nr_running)
> 
> Hummm, didn't we just add_nr_running(rq, 1) before gettng here?

Oh, good catch, let me run some tests to see what happens here. :)

But looking at the code, it seems that we definitely need to move
add_nr_running() after this part.

Thanks!
-Andrea

> 
> > +			dl_server_update_idle_time(rq, rq->curr, &rq->ext_server);
> > +
> > +		/* Start dl_server if this is the first task being enqueued */
> > +		dl_server_start(&rq->ext_server);
> > +	}
> > +
> 
> Thanks,
> Juri
>