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

[Frederic Weisbecker <frederic@kernel.org>]

On Wed, Dec 21, 2022 at 11:26:22AM -0800, Paul E. McKenney wrote:
> On Sat, Dec 17, 2022 at 01:02:03PM +0000, Zhang, Qiang1 wrote:
> commit 5815245b56229198d5f5d0c48c26d3c0c53ee222
> Author: Zqiang <qiang1.zhang@intel.com>
> Date:   Wed Dec 21 11:15:43 2022 -0800
> 
>     rcu: Remove redundant call to rcu_boost_kthread_setaffinity()
>     
>     The rcu_boost_kthread_setaffinity() function is invoked at
>     rcutree_online_cpu() and rcutree_offline_cpu() time, early in the online
>     timeline and late in the offline timeline, respectively.  It is also
>     invoked from rcutree_dead_cpu(), however, in the absence of userspace
>     manipulations (for which userspace must take responsibility), this call
>     is redundant with that from rcutree_offline_cpu().  This redundancy can
>     be demonstrated by printing out the relevant cpumasks
>     
>     This commit therefore removes the call to rcu_boost_kthread_setaffinity()
>     from rcutree_dead_cpu().
>     
>     Signed-off-by: Zqiang <qiang1.zhang@intel.com>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

Thanks!


> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index c3255d27286c4..a6181b76fd5ab 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4165,15 +4165,10 @@ 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;