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

[Joel Fernandes]

On Thu, Aug 18, 2022 at 1:23 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> [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.

I got somewhere with this. It looks like queuing CBs as lazy CBs
instead of normal CBs, are triggering expedited stalls during the boot
process:

  39.949198] rcu: INFO: rcu_preempt detected expedited stalls on
CPUs/tasks: { } 28 jiffies s: 69 root: 0x0/.

No idea how/why lazy RCU CBs would be related to expedited GP issues,
but maybe something hangs and causes that side-effect.

initcall_debug did not help, as it seems initcalls all work fine, and
then 8 seconds after the boot, it starts slowing down a lot, followed
by the RCU stall messages. As a next step I'll enable ftrace during
the boot to see if I can get more insight. But I believe, its not the
FS layer, the FS layer just triggers lazy CBs, but there is something
wrong with the core lazy-RCU work itself.

This kernel is 5.19-rc4. I'll also try to rebase ChromeOS on more
recent kernels and debug.

Thanks,

 - Joel