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

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Message-ID: <Yw4+g/0yEf7fpHrh@pc636>
Date:   Tue, 30 Aug 2022 18:44:51 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.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

Hello, Frederic.

> 
> Although who knows, may be some periodic file operation while idle are specific
> to Android. I'll try to trace lazy callbacks while idle and the number of grace
> periods associated.
> 
> 
Everything related to lazy call-backs is about not waking "nocb"
kthreads in order to offload one or i should say few callbacks
because it is more or less useless. Currently if incoming callback
is the only one, it will kick a GP whereas a GP will kick nocb_kthread
to offload.

In "light" loaded test cases, especially where a power drain is a key
thing, such light load may lead to some kind of "noise" produced by the
RCU core, i.e. kicking idle CPUs, thus wasting power. On our ARM devices
it is not painful, but there is a small power gain and it is visible.
For other systems, like Joel measures for Intel SoC it is more visible,
because of a power cost getting in/out of isle states.

This is what i see on my setup.

--
Uladzislau Rezki