linux-kernel - Re: [PATCH RFC] sched: Disable DL server if sysctl_sched_rt

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Message-ID: <20250304234744.GA2283516@joelnvbox>
Date: Tue, 4 Mar 2025 18:47:44 -0500
From: Joel Fernandes <joelagnelf@...dia.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>, paulmck@...nel.org,
	rcu@...r.kernel.org
Subject: Re: [PATCH RFC] sched: Disable DL server if sysctl_sched_rt_runtime
 is -1

On Tue, Mar 04, 2025 at 03:06:32PM -0500, Steven Rostedt wrote:
> On Tue,  4 Mar 2025 15:01:16 -0500
> Joel Fernandes <joelagnelf@...dia.com> wrote:
> 
> > Currently, RCU boost testing in rcutorture is broken because it relies on
> > having RT throttling disabled. This means the test will always pass (or
> > rarely fail). This occurs because recently, RT throttling was replaced
> > by DL server which boosts CFS tasks even when rcutorture tried to
> > disable throttling (see rcu_torture_disable_rt_throttle()).
> > 
> > Therefore this patch prevents DL server from starting when RC torture
> > sets the sysctl_sched_rt_runtime to -1.
> > 
> > With this patch, boosting in TREE09 fails more than 50% of boost attempts
> > making the test more useful.
> > 
> > Also add a check of this to task_non_contending() because otherwise it
> > throws a warning (in the case when DL server was already started before
> > rcutorture started).
> > 
> 
> Hmm, I wonder if dl_server caused a regression. That is, disabling rt
> throttling should allow RT tasks to starve anything it wants. And some RT
> applications rely on this.
> 
> Should this include a Fixes and Cc stable?

Yeah that makes sense to me, I'll include the Fixes tag in the v2.

thanks,

 - Joel