linux-kernel - Re: [RFC PATCH v3] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too

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Message-ID: <20220427115020.kyaxc5j67lq5zrfq@ava.usersys.com>
Date:   Wed, 27 Apr 2022 12:50:20 +0100
From:   Aaron Tomlin <atomlin@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Christoph Lameter <cl@...two.de>, frederic@...nel.org,
        tglx@...utronix.de, mingo@...nel.org, pauld@...hat.com,
        neelx@...hat.com, oleksandr@...alenko.name,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH v3] tick/sched: Ensure quiet_vmstat() is called when
 the idle tick was stopped too

On Mon 2022-04-25 16:21 -0300, Marcelo Tosatti wrote:
> > Hi Peter, Christoph,
> > 
> > Indeed. Which was why I decided, initially, against the general-purpose
> > case/or approach. Personally, I would prefer to keep this somewhat
> > restrictive to nohz.
> 
> Is there anything that prevents a nohz full CPU from running an
> application with short and frequent idling?

Hi Marcelo,

I'm not sure I understand the question; albeit, if I understand correctly,
yes: the scheduling-clock tick, if it was stopped.
Yet I believe this behaviour is correct. Consider the following example:

  When a CFS task is moved/or migrated to a nohz_full CPU that was
  previously idle and had its tick stopped, if its the only task on the
  run-queue then it is possible that the idle task may not restart the
  tick (see __tick_nohz_full_update_tick()).  Thus once the CFS task exits
  manual intervention i.e. a reschedule IPI to wake the idle task, would be
  required to run again, on the same CPU.

Kind regards,

-- 
Aaron Tomlin