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

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Date:   Fri, 19 Aug 2022 14:17:32 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        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>,
        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

On Fri, Aug 19, 2022 at 2:14 PM Joel Fernandes <joel@...lfernandes.org> wrote:
[..]
> >> Things are much better with the following change. However, this brings
> >> me to a question about lock-contention based or any deferring and boot time.
> >>
> >> If you have a path like selinux doing a synchronize_rcu(), shouldn't we
> >> skip the jiffie waiting for the bypass timer? Otherwise things
> >> synchronously waiting will slow down more than usual. Maybe bypassing
> >> should not be done for any case until boot up is done. I'm curious to
> >> see if that improves boot time.
> >
> > Why not simply disable laziness at boot time and enable it only after
> > booting is complete?  The exiting rcupdate.rcu_normal_after_boot kernel
> > boot parameter uses a similar scheme.
>
> That sounds like the right thing to good, but unfortunately it wont help
> this problem. The boot time issue happens after init has started. So the
> OS is still "booting" even though the kernel has.
>
> Also the problem can happen after boot as well, like if RCU
> lazy/non-lazy callbacks come back to back quickly, or so.
>
> But yes nonetheless, I can see the value of disabling it till the
> in-kernel boot completets.

My mail client is acting weird. I meant to add to this, I wonder if
there is a way other subsystems detect when userspace boots using some
heuristic?

Thanks,

 -  Joel