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

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Message-ID: <7c164cfb-534d-e5eb-f326-a37de9268e6e@joelfernandes.org>
Date:   Tue, 30 Aug 2022 14:53:07 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Uladzislau Rezki <urezki@...il.com>,
        Frederic Weisbecker <frederic@...nel.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>,
        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 8/30/2022 12:44 PM, Uladzislau Rezki wrote:
> 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.

Indeed! And Intel (Rushikesh) reminded me today that it has become more
important than before to not disturb idle CPUs, because more and more pieces of
hardware are moving into SoCs. So if the whole SoC cannot go into deeper sleep
state (package C-state) because of the CPU getting disturbed, then that's lesser
power savings than say in the not-so-recent past. Rushikesh could shed more
light on that fact.

Thanks,

 - Joel