linux-kernel - Re: [RFC PATCH 0/2] Disable RT-throttling for idle-inject threads

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Date: Wed, 10 Apr 2024 15:29:48 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Daniel Bristot de Oliveira <bristot@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Atul Kumar Pant <quic_atulpant@...cinc.com>, mingo@...hat.com, 
	juri.lelli@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org, 
	bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, rafael@...nel.org, 
	daniel.lezcano@...aro.org, kernel@...cinc.com, linux-kernel@...r.kernel.org, 
	linux-pm@...r.kernel.org
Subject: Re: [RFC PATCH 0/2] Disable RT-throttling for idle-inject threads

On Wed, 10 Apr 2024 at 14:24, Daniel Bristot de Oliveira
<bristot@...hat.com> wrote:
>
> On 4/10/24 13:46, Peter Zijlstra wrote:
> > Also, we'll be replacing the throttling code with DL servers 'soonish'
> > at which point all this will stop working anyway, since DL will preempt
> > anything FIFO, including your idle injection crud.

The DL server could make things better for idle injection as it will
let the idle RT threads running on the CPU as long as there is no cfs
thread enqueued whereas the current rt throttling force throttling RT
even if there is no enqueued cfs task. So as long as you have been
able to move things on other cpus, your idle rt thread should keep
running.

>
> +1
>
> also, given that the code spins with preempt disabled, with dl server it could
> even become a non-rt thread...
>
> FIFO RUNNING
>         DL_SERVER activates
>                 their loop
>                         disables preemption()
>                         run()
>                         enable preemption()
>         DL_SERVE throttled
> FIFO BACK
>
> So, there will be no need for this busy loop to be RT.
>
> Anyways, it breaks RT and DL if it keeps running for too long... It can
> also cause complaints like RCU stalls and loong wait on locks, e.g., on
> kworkers...
>
> -- Daniel
>
>
>
>
>