linux-kernel - Re: [PATCH] rcu/nocb: Delete local variable 'need_rcu_nocb_mask' in rcu_init

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Message-ID: <20220513150556.GH1790663@paulmck-ThinkPad-P17-Gen-1>
Date:   Fri, 13 May 2022 08:05:56 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Zhen Lei <thunder.leizhen@...wei.com>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        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,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu/nocb: Delete local variable 'need_rcu_nocb_mask' in
 rcu_init_nohz()

On Tue, Apr 26, 2022 at 03:36:26PM +0800, Zhen Lei wrote:
> The local variable 'need_rcu_nocb_mask' is true only if CONFIG_NO_HZ_FULL
> is defined. So branch "if (need_rcu_nocb_mask)" can be moved within the
> scope of "#if defined(CONFIG_NO_HZ_FULL)". At this point, using variable
> 'need_rcu_nocb_mask' is not necessary, so delete it.
> 
> No functional changes, but the code looks a little more concise.
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>

First, please accept my apologies for the late reply and for the
overly active spam filters.

One question below.

							Thanx, Paul

> ---
>  kernel/rcu/tree_nocb.h | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 636d0546a4e932e..1e334e217f0afb7 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1165,15 +1165,10 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
>  void __init rcu_init_nohz(void)
>  {
>  	int cpu;
> -	bool need_rcu_nocb_mask = false;
>  	struct rcu_data *rdp;
>  
>  #if defined(CONFIG_NO_HZ_FULL)
> -	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> -		need_rcu_nocb_mask = true;
> -#endif /* #if defined(CONFIG_NO_HZ_FULL) */
> -
> -	if (need_rcu_nocb_mask) {

Could you please test this on a kernel built with CONFIG_NO_HZ_FULL=n
and CONFIG_RCU_NOCB_CPU=y?  If that works, please add an explanation
of why it works to the commit log above and repost the patch.

> +	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_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");
> @@ -1182,6 +1177,7 @@ void __init rcu_init_nohz(void)
>  		}
>  		rcu_nocb_is_setup = true;
>  	}
> +#endif /* #if defined(CONFIG_NO_HZ_FULL) */
>  
>  	if (!rcu_nocb_is_setup)
>  		return;
> -- 
> 2.26.0.106.g9fadedd
>