linux-kernel - Re: [PATCH v2] sched/fair: Disable DL server on rcu_torture_disable_rt

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Message-ID: <Z8lsX0GDrx7Pa8vd@jlelli-thinkpadt14gen4.remote.csb>
Date: Thu, 6 Mar 2025 09:35:27 +0000
From: Juri Lelli <juri.lelli@...hat.com>
To: Joel Fernandes <joelagnelf@...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>, stable@...r.kernel.org,
	"Paul E . McKenney" <paulmck@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/fair: Disable DL server on
 rcu_torture_disable_rt_throttle()

Hi Joel,

On 05/03/25 20:10, Joel Fernandes 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()). However, the
> systctl_sched_rt_runtime variable is not considered thus still allowing
> RT tasks to be preempted by CFS tasks.
> 
> Therefore this patch prevents DL server from starting when RCU torture
> sets the sysctl_sched_rt_runtime to -1.
> 
> With this patch, boosting in TREE09 fails reliably if RCU_BOOST=n.
> 
> Steven also mentioned that this could fix RT usecases where users do not
> want DL server to be interfering.
> 
> Cc: stable@...r.kernel.org
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Fixes: cea5a3472ac4 ("sched/fair: Cleanup fair_server")
> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
> ---
> v1->v2:
> 	Updated Fixes tag (Steven)
> 	Moved the stoppage of DL server to fair (Juri)

I think what I suggested/wondered (sorry if I wasn't clear) is that we
might need a link between sched_rt_runtime and the fair_server per-cpu
runtime under sched/debug (i.e., sched_fair_write(), etc), otherwise one
can end up with DL server disabled and still non zero runtime on the
debug interface. This is only if we want to make that link, though;
which I am not entirely sure it is something we want to do, as we will
be stuck with an old/legacy interface if we do. Peter?

Thanks,
Juri