Re: [PATCH 22/54] rcu: replace cpumask_weight with cpumask_empty where appropriate

[Paul E. McKenney]

On Sun, Jan 23, 2022 at 10:38:53AM -0800, Yury Norov wrote:
> In some places, RCU code calls cpumask_weight() to check if any bit of a
> given cpumask is set. We can do it more efficiently with cpumask_empty()
> because cpumask_empty() stops traversing the cpumask as soon as it finds
> first set bit, while cpumask_weight() counts all bits unconditionally.
> 
> Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>

Good point!  Queued and pushed, thank you!

							Thanx, Paul

> ---
>  kernel/rcu/tree_nocb.h   | 4 ++--
>  kernel/rcu/tree_plugin.h | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index eeafb546a7a0..f83c7b1d6110 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1169,7 +1169,7 @@ void __init rcu_init_nohz(void)
>  	struct rcu_data *rdp;
>  
>  #if defined(CONFIG_NO_HZ_FULL)
> -	if (tick_nohz_full_running && cpumask_weight(tick_nohz_full_mask))
> +	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
>  		need_rcu_nocb_mask = true;
>  #endif /* #if defined(CONFIG_NO_HZ_FULL) */
>  
> @@ -1348,7 +1348,7 @@ static void __init rcu_organize_nocb_kthreads(void)
>   */
>  void rcu_bind_current_to_nocb(void)
>  {
> -	if (cpumask_available(rcu_nocb_mask) && cpumask_weight(rcu_nocb_mask))
> +	if (cpumask_available(rcu_nocb_mask) && !cpumask_empty(rcu_nocb_mask))
>  		WARN_ON(sched_setaffinity(current->pid, rcu_nocb_mask));
>  }
>  EXPORT_SYMBOL_GPL(rcu_bind_current_to_nocb);
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index c5b45c2f68a1..0dc0c8d6717c 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -1215,7 +1215,7 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
>  		    cpu != outgoingcpu)
>  			cpumask_set_cpu(cpu, cm);
>  	cpumask_and(cm, cm, housekeeping_cpumask(HK_FLAG_RCU));
> -	if (cpumask_weight(cm) == 0)
> +	if (cpumask_empty(cm))
>  		cpumask_copy(cm, housekeeping_cpumask(HK_FLAG_RCU));
>  	set_cpus_allowed_ptr(t, cm);
>  	free_cpumask_var(cm);
> -- 
> 2.30.2
>