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

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Date:   Thu, 12 Jan 2023 07:29:13 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     "Zhang, Qiang1" <qiang1.zhang@...el.com>,
        "quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rcu: Remove duplicate settings for rcu boost kthreads
 affinity

On Thu, Jan 12, 2023 at 04:07:07PM +0100, Frederic Weisbecker wrote:
> 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@...el.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@...el.com>
> >     Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> 
> Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Thank you!  I will apply this on my next rebase.

							Thanx, Paul

> 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;