linux-kernel - Re: [PATCH] sched/idle: disable tick in idle=poll idle entry

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Message-ID: <ZIMu/GzBZQ1EmiZ0@tpad>
Date:   Fri, 9 Jun 2023 10:54:04 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/idle: disable tick in idle=poll idle entry

On Thu, Jun 08, 2023 at 10:34:00AM +0200, Peter Zijlstra wrote:
> On Wed, Jun 07, 2023 at 10:10:46PM -0300, Marcelo Tosatti wrote:
> > 
> > Commit a5183862e76fdc25f36b39c2489b816a5c66e2e5 
> > ("tick/nohz: Conditionally restart tick on idle exit") allows
> > a nohz_full CPU to enter idle and return from it with the 
> > scheduler tick disabled (since the tick might be undesired noise).
> > 
> > The idle=poll case still unconditionally restarts the tick when entering
> > idle.
> > 
> > To reduce the noise for that case as well, stop the tick when entering
> > idle, for the idle=poll case.
> > 
> > tick_nohz_idle_exit (called if the NEED_RESCHED bit is set) 
> > is responsible for re-enabling the tick when necessary. 
> 
> Who cares and why?

Systems that use isolcpus/nohz_full, and use idle=poll to avoid sleeping.
For example:

https://github.com/intel/FlexRAN/blob/master/README.md

More specifically, the case which HLTs (via nanosleep usually) is:

https://www.isa.org/intech-home/2022/february-2022/departments/what-you-may-not-know-about-softplcs

Can add this to the next version of the patch (unless you have any other
comments).

Thanks.