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

[Paul E. McKenney]

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?

							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
>