Re: [RFC 06/10] rcu/hotplug: Make rcutree_dead_cpu() parallel

From: Joel Fernandes <joel@joelfernandes.org>
To: Pingfan Liu <kernelfans@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>, rcu <rcu@vger.kernel.org>,
	"Paul E. McKenney" <paulmck@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>,
	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>
Subject: Re: [RFC 06/10] rcu/hotplug: Make rcutree_dead_cpu() parallel
Date: Mon, 22 Aug 2022 23:14:29 -0400	[thread overview]
Message-ID: <CAEXW_YTJGqmi15itkPBNt_=Pj6PNG7aXWxq02UWQZ7rzj2mTiA@mail.gmail.com> (raw)
In-Reply-To: <YwQzyMlY6fa2WrM5@piliu.users.ipa.redhat.com>

On Mon, Aug 22, 2022 at 9:56 PM Pingfan Liu <kernelfans@gmail.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@gmail.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@gmail.com>
> > > Cc: "Paul E. McKenney" <paulmck@kernel.org>
> > > Cc: Frederic Weisbecker <frederic@kernel.org>
> > > Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> > > Cc: Josh Triplett <josh@joshtriplett.org>
> > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > > Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> > > Cc: Joel Fernandes <joel@joelfernandes.org>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > Cc: Steven Price <steven.price@arm.com>
> > > Cc: "Peter Zijlstra
> > > Cc: Mark Rutland <mark.rutland@arm.com>
> > > Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> > > Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
> > > To: linux-kernel@vger.kernel.org
> > > To: rcu@vger.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
> > >