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

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Date:   Thu, 1 Sep 2022 13:29:47 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Uladzislau Rezki <urezki@...il.com>
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>,
        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 Tue, Aug 30, 2022 at 06:44:51PM +0200, 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.

Not sure this is only about not waking "nocb" kthreads. The grace period
kthread is also awaken in !NOCB and has quite some work to do. And there,
having a server expands the issue because you may have a lot of CPUs's extended
quiescent states to check.

Also in !NOCB, pending callbacks retain the timer tick of a CPU (see
rcu_needs_cpu()), and cpuidle relies on the tick to be stopped before
allowing the CPU into low power mode. So a lazy callback may delay a CPU from
entering into low power mode for a few milliseconds.

And I can observe those retained ticks on my idle box.

Thanks.