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

[Pingfan Liu]

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.

[1]: https://lore.kernel.org/linux-arm-kernel/20220822021520.6996-3-kernelfans@xxxxxxxxx/T/#mf62352138d7b040fdb583ba66f8cd0ed1e145feb

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
> >