linux-kernel - Re: [RFC][PATCH 1/5] sched/deadline: Less agressive dl

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Message-ID: <20250613094328.GG2278213@noisy.programming.kicks-ass.net>
Date: Fri, 13 Jun 2025 11:43:28 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Juri Lelli <juri.lelli@...hat.com>
Cc: mingo@...hat.com, vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, clm@...a.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 1/5] sched/deadline: Less agressive dl_server
 handling

On Tue, Jun 03, 2025 at 06:03:12PM +0200, Juri Lelli wrote:
> Hi,
> 

> > @@ -1684,6 +1689,24 @@ void dl_server_stop(struct sched_dl_enti
> >  	dl_se->dl_server_active = 0;
> >  }
> >  
> > +static bool dl_server_stopped(struct sched_dl_entity *dl_se)
> > +{
> > +	if (!dl_se->dl_server_active)
> > +		return false;
> > +
> > +	if (dl_se->dl_server_idle) {
> > +		__dl_server_stop(dl_se);
> > +		return true;
> > +	}
> > +
> > +	dl_se->dl_server_idle = 1;
> > +	return false;
> > +}
> > +
> > +void dl_server_stop(struct sched_dl_entity *dl_se)
> > +{
> > +}
> 
> What if we explicitly set the server to idle (instead of ignoring the
> stop) where this gets called in dequeue_entities()?

That would break thing; we want to detect if it was ever !idle in the
period.

> Also, don't we need to actually stop the server if we are changing its
> parameters from sched_fair_server_write()?

Quite - let me just remove the offending callsites them.

Would this explain this massive regression 0day reported here? Seems
weird.

Anyway, let me go update the patch.