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In-Reply-To: <20220829204202.GQ6159@paulmck-ThinkPad-P17-Gen-1>
From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Mon, 29 Aug 2022 16:48:25 -0400
Message-ID: <CAEXW_YSwkSi0UfBUrK=4gyMPC-R7pvQbreWOC3DcQqmLgp3aFQ@mail.gmail.com>
Subject: Re: [PATCH v4 00/14] Implement call_rcu_lazy() and miscellaneous fixes
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Frederic Weisbecker <frederic@kernel.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Rushikesh S Kadam <rushikesh.s.kadam@intel.com>,
        "Uladzislau Rezki (Sony)" <urezki@gmail.com>,
        Neeraj upadhyay <neeraj.iitr10@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        rcu <rcu@vger.kernel.org>,
        Vineeth Pillai <vineeth@bitbyteword.org>
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On Mon, Aug 29, 2022 at 4:42 PM Paul E. McKenney <paulmck@kernel.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@kernel.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...

As you said, let us see v5 and how we want to proceed from there (as
it is not too far from posting) . I do appreciate Frederick's review
and valid concerns.

Thanks,

- Joel