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

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Message-ID: <CAFgQCTuNw3sdO=X1KNHkTZW8YvK8xo4bmTxyN_uJ9=kkWOW=zw@mail.gmail.com>
Date:   Mon, 5 Sep 2022 11:53:52 +0800
From:   Pingfan Liu <kernelfans@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
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 Thu, Sep 1, 2022 at 12:15 AM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> 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.)
> >

Sorry to reply late, since I am interrupted by some other things.
I have took a different way and posted a series ([PATCH 1/3] rcu:
remove redundant cpu affinity setting during teardown) for that on
https://lore.kernel.org/rcu/20220905033852.18988-1-kernelfans@gmail.com/T/#t

Besides, for the integration of the concurrency cpu hot-removing into
the rcu torture test, I begin to do it.

> > There are probably more where that one came from.  ;-)
>
> And here is one more from this week's session.
>

Thanks for the update.

> 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.
>

I will see what I can do here.

> For more detail, please see the Google document:
>
> https://docs.google.com/document/d/1jymsaCPQ1PUDcfjIKm0UIbVdrJAaGX-6cXrmcfm0PRU/edit?usp=sharing
>

Have read it and hope that both online and offline concurrency can
come to true in near future.

Thanks,

    Pingfan