linux-kernel - Re: [PATCH 22/54] rcu: replace cpumask_weight with cpumask

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Message-ID: <20220124000734.GB4285@paulmck-ThinkPad-P17-Gen-1>
Date:   Sun, 23 Jan 2022 16:07:34 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Laight <David.Laight@...lab.com>,
        Joe Perches <joe@...ches.com>, Dennis Zhou <dennis@...nel.org>,
        Emil Renner Berthing <kernel@...il.dk>,
        Nicholas Piggin <npiggin@...il.com>,
        Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Alexey Klimov <aklimov@...hat.com>,
        linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: Re: [PATCH 22/54] rcu: replace cpumask_weight with cpumask_empty
 where appropriate

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@...il.com>

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
>