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SubjectRe: [RFC 06/10] rcu/hotplug: Make rcutree_dead_cpu() parallel
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@gmail.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?

> [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.

Thoughts?

Thanx, Paul

> Thanks,
>
> Pingfan
>
>
> > Thanx, Paul
> >
> > > 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
> > >

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