linux-kernel - Re: [PATCH] rcu: Remove duplicate settings for rcu boost kthreads affinity

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Date:   Fri, 16 Dec 2022 17:09:19 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Zqiang <qiang1.zhang@...el.com>
Cc:     frederic@...nel.org, quic_neeraju@...cinc.com,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Remove duplicate settings for rcu boost kthreads
 affinity

On Thu, Dec 15, 2022 at 05:00:29PM +0800, Zqiang wrote:
> Currently, when CPUs going offline, the rcu boost kthreads CPU
> affinity has been reset in rcutree_offline_cpu(), therefore this
> commit remove duplicate settings in rcutree_dead_cpu().
> 
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>

One question...

Given that rcutree_offline_cpu() causes rcu_boost_kthread_setaffinity()
to be invoked with the number of the outgoing CPU, but rcutree_dead_cpu()
instead passes in -1, are these two invocations really redundant?

							Thanx, paul

> ---
>  kernel/rcu/tree.c | 5 -----
>  1 file changed, 5 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 65f3dd2fd3ae..20de83ed0c7e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4122,15 +4122,10 @@ static void rcu_cleanup_dead_rnp(struct rcu_node *rnp_leaf)
>   */
>  int rcutree_dead_cpu(unsigned int cpu)
>  {
> -	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> -	struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */
> -
>  	if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
>  		return 0;
>  
>  	WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
> -	/* Adjust any no-longer-needed kthreads. */
> -	rcu_boost_kthread_setaffinity(rnp, -1);
>  	// Stop-machine done, so allow nohz_full to disable tick.
>  	tick_dep_clear(TICK_DEP_BIT_RCU);
>  	return 0;
> -- 
> 2.25.1
>