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In-Reply-To: <20220819182645.GQ2125313@paulmck-ThinkPad-P17-Gen-1>
From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Fri, 19 Aug 2022 14:29:29 -0400
Message-ID: <CAEXW_YT0==us-Suq74W5HV-ZbykmAY3cGoNDU2KQbTG3ZksgjQ@mail.gmail.com>
Subject: Re: [PATCH v3 resend 4/6] fs: Move call_rcu() to call_rcu_lazy() in
 some paths
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Rushikesh S Kadam <rushikesh.s.kadam@intel.com>,
        "Uladzislau Rezki (Sony)" <urezki@gmail.com>,
        Neeraj upadhyay <neeraj.iitr10@gmail.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>, rcu <rcu@vger.kernel.org>
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On Fri, Aug 19, 2022 at 2:26 PM Paul E. McKenney <paulmck@kernel.org> 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@joelfernandes.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?
>
> 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