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

[Joel Fernandes]

On Fri, Aug 19, 2022 at 2:26 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Fri, Aug 19, 2022 at 02:17:32PM -0400, Joel Fernandes wrote:
> > On Fri, Aug 19, 2022 at 2:14 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> 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?
>
> I don't know of one, but I bet that ChromeOS has ways.  If nothing else,
> might you add a sysfs write to one of the boot-up phases?

Yes, that's possible :) Thanks, I will consider this idea.

Thanks,

 - Joel