linux-kernel - Re: [PATCH 4/9] rcu: Introduce lazy queue's own qhimark

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Message-ID: <ZKVlzOuQ0HyhSZ3L@localhost.localdomain>
Date:   Wed, 5 Jul 2023 14:45:00 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, rcu <rcu@...r.kernel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Giovanni Gherdovich <ggherdovich@...e.cz>
Subject: Re: [PATCH 4/9] rcu: Introduce lazy queue's own qhimark

Le Sat, Jun 03, 2023 at 01:23:38AM +0000, Joel Fernandes a écrit :
> On Wed, May 31, 2023 at 12:17:31PM +0200, Frederic Weisbecker wrote:
> > The lazy and the regular bypass queues share the same thresholds in
> > terms of number of callbacks after which a flush to the main list is
> > performed: 10 000 callbacks.
> > 
> > However lazy and regular bypass don't have the same purposes and neither
> > should their respective thresholds:
> > 
> > * The bypass queue stands for relieving the main queue in case of a
> >   callback storm. It makes sense to allow a high number of callbacks to
> >   pile up before flushing to the main queue, especially as the life
> >   cycle for this queue is very short (1 jiffy).
> 
> Sure.
> 
> > 
> > * The lazy queue aims to spare wake ups and reduce the number of grace
> >   periods. There it doesn't make sense to allow a huge number of
> >   callbacks before flushing so as not to introduce memory pressure,
> >   especially as the life cycle for this queue is very long (10
> >   seconds).
> 
> It does make sense as we have a shrinker, and it is better to avoid RCU
> disturbing the system unwantedly when there's lots of memory lying around.
> 
> > 
> > For those reasons, set the default threshold for the lazy queue to
> > 100.
> 
> I am OK with splitting the qhimark, but this lazy default is too low. In
> typical workloads on ChromeOS, we see 1000s of callback even when CPU
> utilization is low. So considering that, we would be flushing all the time.
> 
> Eventually I want the mm subsystem to tell us when flushing is needed so we
> could flush sooner at that time if really needed, but for now we have a
> shrinker so it should be OK. Is there a reason the shrinker does not work for
> you?

So you mean dynamically adapting the lazy qhimark on top of shrinker calls,
right? That would make sense indeed.

Thanks.