linux-kernel - Re: [PATCH v3 resend 4/6] fs: Move call_rcu() to call_rcu

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Message-ID: <CAEXW_YQuGga9Eivq4G6o1XjvPn-nMMDiM8FOY6HXJTMzwv1Emg@mail.gmail.com>
Date:   Thu, 18 Aug 2022 13:23:07 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Rushikesh S Kadam <rushikesh.s.kadam@...el.com>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        Neeraj upadhyay <neeraj.iitr10@...il.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>, rcu <rcu@...r.kernel.org>
Subject: Re: [PATCH v3 resend 4/6] fs: Move call_rcu() to call_rcu_lazy() in
 some paths

[Sorry, adding back the CC list]

On Mon, Aug 8, 2022 at 11:45 PM Joel Fernandes (Google)
<joel@...lfernandes.org> wrote:
>
> This is required to prevent callbacks triggering RCU machinery too
> quickly and too often, which adds more power to the system.
>
> When testing, we found that these paths were invoked often when the
> system is not doing anything (screen is ON but otherwise idle).

Unfortunately, I am seeing a slow down in ChromeOS boot performance
after applying this particular patch. It is the first time I could
test ChromeOS boot times with the series since it was hard to find a
ChromeOS device that runs the upstream kernel.

Anyway, Vlad, Neeraj, do you guys also see slower boot times with this
patch? I wonder if the issue is with wake up interaction with the nocb
GP threads.

We ought to disable lazy RCU during boot since it would have little
benefit anyway. But I am also concerned about some deeper problem I
did not catch before.

I'll look into tracing the fs paths to see if I can narrow down what's
causing it. Will also try a newer kernel, I am currently testing on
5.19-rc4.

Thanks,

 - Joel