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From:   Pingfan Liu <kernelfans@gmail.com>
Date:   Wed, 24 Aug 2022 21:53:11 +0800
Message-ID: <CAFgQCTup0uTqnKi79Tu+5Q0POYVdcE4UkGes8KfHXBd6VR552A@mail.gmail.com>
Subject: Re: [RFC 06/10] rcu/hotplug: Make rcutree_dead_cpu() parallel
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>, rcu <rcu@vger.kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Price <steven.price@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>, boqun.feng@gmail.com
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On Tue, Aug 23, 2022 at 11:01 AM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Tue, Aug 23, 2022 at 09:50:56AM +0800, Pingfan Liu wrote:
> > On Sun, Aug 21, 2022 at 07:45:28PM -0700, Paul E. McKenney wrote:
> > > On Mon, Aug 22, 2022 at 10:15:16AM +0800, Pingfan Liu wrote:
> > > > In order to support parallel, rcu_state.n_online_cpus should be
> > > > atomic_dec()
> > > >
> > > > Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> > >
> > > I have to ask...  What testing have you subjected this patch to?
> > >
> >
> > This patch subjects to [1]. The series aims to enable kexec-reboot in
> > parallel on all cpu. As a result, the involved RCU part is expected to
> > support parallel.
>
> I understand (and even sympathize with) the expectation.  But results
> sometimes diverge from expectations.  There have been implicit assumptions
> in RCU about only one CPU going offline at a time, and I am not sure
> that all of them have been addressed.  Concurrent CPU onlining has
> been looked at recently here:
>
> https://docs.google.com/document/d/1jymsaCPQ1PUDcfjIKm0UIbVdrJAaGX-6cXrmcfm0PRU/edit?usp=sharing
>
> You did us atomic_dec() to make rcu_state.n_online_cpus decrementing be
> atomic, which is good.  Did you look through the rest of RCU's CPU-offline
> code paths and related code paths?
>

I went through those codes at a shallow level, especially at each
cpuhp_step hook in the RCU system.

But as you pointed out, there are implicit assumptions about only one
CPU going offline at a time, I will chew the google doc which you
share.  Then I can come to a final result.

> > [1]: https://lore.kernel.org/linux-arm-kernel/20220822021520.6996-3-kernelfans@gmail.com/T/#mf62352138d7b040fdb583ba66f8cd0ed1e145feb
>
> Perhaps I am more blind than usual today, but I am not seeing anything
> in this patch describing the testing.  At this point, I am thinking in
> terms of making rcutorture test concurrent CPU offlining parallel
>

Yes, testing results are more convincing in this area.

After making clear the implicit assumptions, I will write some code to
bridge my code and rcutorture test. Since the series is a little
different from parallel cpu offlining. It happens after all devices
are torn down, and there is no way to rollback.

> Thoughts?
>

Need a deeper dive into this field. Hope to bring out something soon.


Thanks,

    Pingfan