linux-kernel - Re: [RFC 06/10] rcu/hotplug: Make rcutree_dead

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Message-ID: <20220831161522.GA2582451@paulmck-ThinkPad-P17-Gen-1>
Date:   Wed, 31 Aug 2022 09:15:22 -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 09:20:50AM -0700, Paul E. McKenney wrote:
> 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.  ;-)

And here is one more from this week's session.

The calls to tick_dep_set() and tick_dep_clear() use atomic operations,
but they operate on a global variable.  This means that the first call
to rcutree_offline_cpu() would enable the tick and the first call to
rcutree_dead_cpu() would disable the tick.  This might be OK, but it
is at the very least bad practice.  There needs to be a counter
mediating these calls.

For more detail, please see the Google document:

https://docs.google.com/document/d/1jymsaCPQ1PUDcfjIKm0UIbVdrJAaGX-6cXrmcfm0PRU/edit?usp=sharing

							Thanx, Paul

> > > > [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