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
>
