Re: [PATCH 3/6] rcu/nocb: Optimize kthreads and rdp initialization

[Neeraj Upadhyay <quic_neeraju@quicinc.com>]

On 11/23/2021 6:07 AM, Frederic Weisbecker wrote:
> Currently cpumask_available() is used to prevent from unwanted
> NOCB initialization. However if neither "rcu_nocbs=" nor "nohz_full="
> parameters are passed but CONFIG_CPUMASK_OFFSTACK=n, the initialization
> path is still taken, running through all sorts of needless operations
> and iterations on an empty cpumask.
> 
> Fix this with relying on a real initialization state instead. This
> also optimize kthreads creation, sparing iteration over all online CPUs
> when nocb isn't initialized.
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Uladzislau Rezki <urezki@gmail.com>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Joel Fernandes <joel@joelfernandes.org>
> ---

Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>

>   kernel/rcu/tree_nocb.h | 24 +++++++++++++++++-------
>   1 file changed, 17 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index e1cb06840454..d8ed3ee47a67 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -60,6 +60,9 @@ static inline bool rcu_current_is_nocb_kthread(struct rcu_data *rdp)
>    * Parse the boot-time rcu_nocb_mask CPU list from the kernel parameters.
>    * If the list is invalid, a warning is emitted and all CPUs are offloaded.
>    */
> +
> +static bool rcu_nocb_is_setup;
> +
>   static int __init rcu_nocb_setup(char *str)
>   {
>   	alloc_bootmem_cpumask_var(&rcu_nocb_mask);
> @@ -67,6 +70,7 @@ static int __init rcu_nocb_setup(char *str)
>   		pr_warn("rcu_nocbs= bad CPU range, all CPUs set\n");
>   		cpumask_setall(rcu_nocb_mask);
>   	}
> +	rcu_nocb_is_setup = true;
>   	return 1;
>   }
>   __setup("rcu_nocbs=", rcu_nocb_setup);
> @@ -1159,13 +1163,17 @@ void __init rcu_init_nohz(void)
>   		need_rcu_nocb_mask = true;
>   #endif /* #if defined(CONFIG_NO_HZ_FULL) */
>   
> -	if (!cpumask_available(rcu_nocb_mask) && need_rcu_nocb_mask) {
> -		if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
> -			pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
> -			return;
> +	if (need_rcu_nocb_mask) {
> +		if (!cpumask_available(rcu_nocb_mask)) {
> +			if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
> +				pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
> +				return;
> +			}
>   		}
> +		rcu_nocb_is_setup = true;
>   	}
> -	if (!cpumask_available(rcu_nocb_mask))
> +
> +	if (!rcu_nocb_is_setup)
>   		return;
>   
>   #if defined(CONFIG_NO_HZ_FULL)
> @@ -1267,8 +1275,10 @@ static void __init rcu_spawn_nocb_kthreads(void)
>   {
>   	int cpu;
>   
> -	for_each_online_cpu(cpu)
> -		rcu_spawn_cpu_nocb_kthread(cpu);
> +	if (rcu_nocb_is_setup) {
> +		for_each_online_cpu(cpu)
> +			rcu_spawn_cpu_nocb_kthread(cpu);
> +	}
>   }
>   
>   /* How many CB CPU IDs per GP kthread?  Default of -1 for sqrt(nr_cpu_ids). */
>