linux-kernel - Re: [PATCH sched_ext/for-6.12] sched_ext: Handle cases where pick_task_scx() is called without preceding balance

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Message-ID: <Zy_Da_IEJYD2ykgj@slm.duckdns.org>
Date: Sat, 9 Nov 2024 10:17:47 -1000
From: Tejun Heo <tj@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: David Vernet <void@...ifault.com>, linux-kernel@...r.kernel.org,
	kernel-team@...a.com
Subject: Re: [PATCH sched_ext/for-6.12] sched_ext: Handle cases where
 pick_task_scx() is called without preceding balance_scx()

Hello, Peter.

On Fri, Sep 06, 2024 at 11:04:20AM +0200, Peter Zijlstra wrote:
> On Thu, Sep 05, 2024 at 03:17:13PM -1000, Tejun Heo wrote:
> > On Thu, Sep 05, 2024 at 06:41:42AM -1000, Tejun Heo wrote:
> > > > @@ -12716,6 +12716,12 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
> > > >  	if (this_rq->cfs.h_nr_running && !pulled_task)
> > > >  		pulled_task = 1;
> > > >  
> > > > +	/*
> > > > +	 * We pulled a task, but it got stolen before we re-acquired rq->lock.
> > > > +	 */
> > > > +	if (!this_rq->cfs.h_nr_running && pulled_task)
> > > > +		pulled_task = 0;
> > > > +
> > > 
> > > Lemme test that.
> > 
> > Did a bit of testing and it seems like it's mostly coming from delayed
> > dequeue handling. pick_next_entity() does this:
> > 
> > 	struct sched_entity *se = pick_eevdf(cfs_rq);
> > 	if (se->sched_delayed) {
> > 		dequeue_entities(rq, se, DEQUEUE_SLEEP | DEQUEUE_DELAYED);
> > 		SCHED_WARN_ON(se->sched_delayed);
> > 		SCHED_WARN_ON(se->on_rq);
> > 		return NULL;
> > 	}
> > 
> > rq->cfs.nr_running includes the number of delay dequeued tasks which aren't
> > really runnable, so it seems like balance_fair() saying yes and
> > pick_next_entity() then hitting a delayed task.
> 
> Duh, yes.
> 
> > Maybe the solution is
> > tracking the number of delayed ones and subtracting that from nr_running?
> 
> That came up yesterday for something else as well. Let me see if I can
> make that happen.
> 
> 
> Anyway, I suppose you keep your patch for now until I've managed to sort
> this out.

This still triggers. I'm going to apply the workaround for now.

Thanks.

-- 
tejun