Re: [PATCH v3 resend 4/6] fs: Move call_rcu() to call_rcu_lazy() in some paths

[Joel Fernandes]

[Sorry, adding back the CC list]

On Mon, Aug 8, 2022 at 11:45 PM Joel Fernandes (Google)
<joel@xxxxxxxxxxxxxxxxx> 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