linux-kernel - Re: [PATCH 1/4] tick/sched: Forward timer even in nohz mode

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Message-ID: <6d18d7772f9ad1f42444ea936243ce958a1345dc.camel@redhat.com>
Date:   Wed, 08 Jan 2020 17:18:09 -0600
From:   Scott Wood <swood@...hat.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] tick/sched: Forward timer even in nohz mode

On Mon, 2020-01-06 at 23:18 +0100, Frederic Weisbecker wrote:
> On Mon, Dec 16, 2019 at 06:22:22PM -0500, Scott Wood wrote:
> > Currently when exiting nohz, the expiry will be forwarded as if we
> > had just run the timer.  If we re-enter nohz before this new expiry,
> > and exit after, this forwarding will happen again.  If this load pattern
> > recurs the tick can be indefinitely postponed.
> 
> I must be missing something but I don't see why that would be a problem.
> Indeed the tick can be indefinitely postponed but that's as long as it's
> not needed. As soon as it's needed (timer callback expired, RCU, ...), the
> tick will be retained and it will eventually fire.

My main concern was the loadavg recording, which does not prevent entering
nohz.  However, if we're going into and out of nohz then the call to
calc_load_nohz_start() in tick_nohz_stop_tick() should take care of it, so I
guess we only need patches 3 and 4, if there's nothing else in the tick that
doesn't either prevent nohz or get handled during the transition
(psi_task_tick?).

-Scott