linux-kernel - Re: [PATCH v4 00/14] Implement call_rcu

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Date:   Tue, 30 Aug 2022 12:57:39 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Rushikesh S Kadam <rushikesh.s.kadam@...el.com>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        Neeraj upadhyay <neeraj.iitr10@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        rcu <rcu@...r.kernel.org>,
        Vineeth Pillai <vineeth@...byteword.org>
Subject: Re: [PATCH v4 00/14] Implement call_rcu_lazy() and miscellaneous
 fixes

On Mon, Aug 29, 2022 at 04:48:25PM -0400, Joel Fernandes wrote:
> On Mon, Aug 29, 2022 at 4:42 PM Paul E. McKenney <paulmck@...nel.org> wrote:
> >
> > On Mon, Aug 29, 2022 at 04:36:40PM -0400, Joel Fernandes wrote:
> > > On Mon, Aug 29, 2022 at 3:46 PM Frederic Weisbecker <frederic@...nel.org> wrote:
> > > > On Mon, Aug 29, 2022 at 12:45:40PM -0400, Joel Fernandes wrote:
> > > > > On 8/29/2022 9:40 AM, Frederic Weisbecker wrote:
> >
> > [ . .  . ]
> >
> > > > > > 2) NOCB implies performance issues.
> > > > >
> > > > > Which kinds of? There is slightly worse boot times, but I'm guessing that's do
> > > > > with the extra scheduling overhead of the extra threads which is usually not a
> > > > > problem except that RCU is used in the critical path of boot up (on ChromeOS).
> > > >
> > > > I never measured it myself but executing callbacks on another CPUs, with
> > > > context switches and locking can only involve significant performance issues if callbacks
> > > > are frequent. So it's a tradeoff between power and performance.
> > >
> > > In my testing of benchmarks on real systems with 8-16 CPUs, the
> > > performance hit is down in the noise. It is possible though that maybe
> > > one can write a non-realistic synthetic test to force the performance
> > > issues, but I've not seen it in the real world. Maybe on
> > > networking-heavy servers with lots of cores, you'll see it but their
> > > batteries if any would be pretty big :-).
> >
> > To Frederic's point, if you have enough servers, even a 1% decrease in
> > power consumption is a very big deal.  ;-)
> 
> Ah I see Frederick's point now, so basically the claim is that using
> lazy-RCU on servers might make sense to save power because
> CONFIG_RCU_NO_CB_CPU may not be an option there (networking throughput
> and so forth).
> 
> That's a good point indeed...

You got it! And it's not just servers but pretty much everything that may
be idle sometimes. Distros (except android then) don't use rcu_nocbs= by
default. Routers enjoy night bandwith, etc...

Thanks!