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Message-ID: <20220824162050.GA6159@paulmck-ThinkPad-P17-Gen-1>
Date: Wed, 24 Aug 2022 09:20:50 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Pingfan Liu <kernelfans@...il.com>
Cc: LKML <linux-kernel@...r.kernel.org>, rcu <rcu@...r.kernel.org>,
Frederic Weisbecker <frederic@...nel.org>,
Neeraj Upadhyay <quic_neeraju@...cinc.com>,
Josh Triplett <josh@...htriplett.org>,
Steven Rostedt <rostedt@...dmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
Joel Fernandes <joel@...lfernandes.org>,
Thomas Gleixner <tglx@...utronix.de>,
Steven Price <steven.price@....com>,
Mark Rutland <mark.rutland@....com>,
Kuppuswamy Sathyanarayanan
<sathyanarayanan.kuppuswamy@...ux.intel.com>,
"Jason A. Donenfeld" <Jason@...c4.com>, boqun.feng@...il.com
Subject: Re: [RFC 06/10] rcu/hotplug: Make rcutree_dead_cpu() parallel
On Wed, Aug 24, 2022 at 09:53:11PM +0800, Pingfan Liu wrote:
> On Tue, Aug 23, 2022 at 11:01 AM Paul E. McKenney <paulmck@...nel.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@...il.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.
And that is fine, at least as a first step.
> 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.
Boqun Feng, Neeraj Upadhyay, Uladzislau Rezki, and I took a quick look,
and rcu_boost_kthread_setaffinity() seems to need some help. As it
stands, it appears that concurrent invocations of this function from the
CPU-offline path will cause all but the last outgoing CPU's bit to be
(incorrectly) set in the cpumask_var_t passed to set_cpus_allowed_ptr().
This should not be difficult to fix, for example, by maintaining a
separate per-leaf-rcu_node-structure bitmask of the concurrently outgoing
CPUs for that rcu_node structure. (Similar in structure to the
->qsmask field.)
There are probably more where that one came from. ;-)
> > > [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.
Very good, looking forward to seeing what you come up with!
> > Thoughts?
>
> Need a deeper dive into this field. Hope to bring out something soon.
Again, looking forward to seeing what you find!
Thanx, Paul
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