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: <20220427144927.c5as3sa4mheawea5@ava.usersys.com>
Date:   Wed, 27 Apr 2022 15:49:27 +0100
From:   Aaron Tomlin <atomlin@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Christoph Lameter <cl@...two.de>, frederic@...nel.org,
        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 Wed 2022-04-27 16:40 +0200, Thomas Gleixner wrote:
> On Wed, Apr 27 2022 at 12:50, Aaron Tomlin wrote:
> > On Mon 2022-04-25 16:21 -0300, Marcelo Tosatti wrote:
> >> Is there anything that prevents a nohz full CPU from running an
> >> application with short and frequent idling?
> >
> > 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.
> 
> When the task exits and the tick was stopped, why should idle restart
> the tick? There is nothing to do, so what?

Hi Thomas,

Indeed. As per my response, I do not see an issue. Perhaps I misunderstood
Marcelo's question, no?

Kind regards,

-- 
Aaron Tomlin