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

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Message-ID: <CAEXW_YTJGqmi15itkPBNt_=Pj6PNG7aXWxq02UWQZ7rzj2mTiA@mail.gmail.com>
Date:   Mon, 22 Aug 2022 23:14:29 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, rcu <rcu@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...nel.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>,
        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>
Subject: Re: [RFC 06/10] rcu/hotplug: Make rcutree_dead_cpu() parallel

On Mon, Aug 22, 2022 at 9:56 PM Pingfan Liu <kernelfans@...il.com> wrote:
>
> On Mon, Aug 22, 2022 at 02:08:38PM -0400, Joel Fernandes wrote:
> > On Sun, Aug 21, 2022 at 10:16 PM Pingfan Liu <kernelfans@...il.com> wrote:
> > >
> > > In order to support parallel, rcu_state.n_online_cpus should be
> > > atomic_dec()
> >
> > What does Parallel mean? Is that some kexec terminology?
> >
>
> 'Parallel' means concurrent. It is not a kexec terminology, instead,
> should be SMP.

Ah ok! Makes sense. Apologies to be the word-police here, but you
probably could reword it to "In order to support parallel offlining"
or some such.

 - Joel



>
> Thanks,
>
>         Pingfan
>
>
> > Thanks,
> >
> >  - Joel
> >
> > >
> > > Signed-off-by: Pingfan Liu <kernelfans@...il.com>
> > > Cc: "Paul E. McKenney" <paulmck@...nel.org>
> > > Cc: Frederic Weisbecker <frederic@...nel.org>
> > > Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
> > > Cc: Josh Triplett <josh@...htriplett.org>
> > > Cc: Steven Rostedt <rostedt@...dmis.org>
> > > Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > > Cc: Lai Jiangshan <jiangshanlai@...il.com>
> > > Cc: Joel Fernandes <joel@...lfernandes.org>
> > > Cc: Thomas Gleixner <tglx@...utronix.de>
> > > Cc: Steven Price <steven.price@....com>
> > > Cc: "Peter Zijlstra
> > > Cc: Mark Rutland <mark.rutland@....com>
> > > Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
> > > Cc: "Jason A. Donenfeld" <Jason@...c4.com>
> > > To: linux-kernel@...r.kernel.org
> > > To: rcu@...r.kernel.org
> > > ---
> > >  kernel/cpu.c      | 1 +
> > >  kernel/rcu/tree.c | 3 ++-
> > >  2 files changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/cpu.c b/kernel/cpu.c
> > > index 1261c3f3be51..90debbe28e85 100644
> > > --- a/kernel/cpu.c
> > > +++ b/kernel/cpu.c
> > > @@ -1872,6 +1872,7 @@ static struct cpuhp_step cpuhp_hp_states[] = {
> > >                 .name                   = "RCU/tree:prepare",
> > >                 .startup.single         = rcutree_prepare_cpu,
> > >                 .teardown.single        = rcutree_dead_cpu,
> > > +               .support_kexec_parallel = true,
> > >         },
> > >         /*
> > >          * On the tear-down path, timers_dead_cpu() must be invoked
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 79aea7df4345..07d31e16c65e 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -2168,7 +2168,8 @@ int rcutree_dead_cpu(unsigned int cpu)
> > >         if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
> > >                 return 0;
> > >
> > > -       WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
> > > +       /* Hot remove path allows parallel, while Hot add races against remove on lock */
> > > +       atomic_dec((atomic_t *)&rcu_state.n_online_cpus);
> > >         /* Adjust any no-longer-needed kthreads. */
> > >         rcu_boost_kthread_setaffinity(rnp, -1);
> > >         // Stop-machine done, so allow nohz_full to disable tick.
> > > --
> > > 2.31.1
> > >