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

[Paul E. McKenney]

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@xxxxxxxxx>
> > 
> > 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@xxxxxxxxx/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@xxxxxxxxxx>
> > > Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > > Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
> > > Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> > > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > > Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > > Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> > > Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > Cc: Steven Price <steven.price@xxxxxxx>
> > > Cc: "Peter Zijlstra
> > > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > > Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> > > Cc: "Jason A. Donenfeld" <Jason@xxxxxxxxx>
> > > To: linux-kernel@xxxxxxxxxxxxxxx
> > > To: rcu@xxxxxxxxxxxxxxx
> > > ---
> > >  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
> > >