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

[Joel Fernandes]

On Sun, Aug 21, 2022 at 10:16 PM Pingfan Liu <kernelfans@xxxxxxxxx> wrote:
>
> In order to support parallel, rcu_state.n_online_cpus should be
> atomic_dec()

What does Parallel mean? Is that some kexec terminology?

Thanks,

 - Joel

>
> Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
> 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
>