linux-kernel - Re: [PATCH 5/5] rcu/exp: Warn on CPU lagging for too long within hotplug IPI's blindspot

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Message-ID: <6c31d2e2-d37f-4a08-b857-a7ac90de08be@paulmck-laptop>
Date: Wed, 19 Mar 2025 07:04:42 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Boqun Feng <boqun.feng@...il.com>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Neeraj Upadhyay <neeraj.upadhyay@....com>,
	Uladzislau Rezki <urezki@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>, rcu <rcu@...r.kernel.org>
Subject: Re: [PATCH 5/5] rcu/exp: Warn on CPU lagging for too long within
 hotplug IPI's blindspot

On Wed, Mar 19, 2025 at 10:42:38AM +0100, Frederic Weisbecker wrote:
> Le Tue, Mar 18, 2025 at 10:22:33AM -0700, Paul E. McKenney a écrit :
> > On Fri, Mar 14, 2025 at 03:36:42PM +0100, Frederic Weisbecker wrote:
> > > A CPU within hotplug operations can make the RCU exp kworker lagging if:
> > > 
> > > * The dying CPU is running after CPUHP_TEARDOWN_CPU but before
> > >   rcutree_report_cpu_dead(). It is too late to send an IPI but RCU is
> > >   still watching the CPU. Therefore the exp kworker can only wait for
> > >   the target to reach rcutree_report_cpu_dead().
> > > 
> > > * The booting CPU is running after rcutree_report_cpu_starting() but
> > >   before set_cpu_online(). RCU is watching the CPU but it is too early
> > >   to be able to send an IPI. Therefore the exp kworker can only wait
> > >   until it observes the CPU as officially online.
> > > 
> > > Such a lag is expected to be very short. However #VMEXIT and other
> > > hazards can stay on the way. Report long delays, 10 jiffies is
> > > considered a high threshold already.
> > > 
> > > Reported-by: Paul E. McKenney <paulmck@...nel.org>
> > > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > 
> > Same CONFIG_PROVE_RCU question, same conditional:
> > 
> > Reviewed-by: Paul E. McKenney <paulmck@...nel.org>
> 
> I don't have a strong opinion whether to keep this warning unconditional.
> Perhaps this can depend on CONFIG_PROVE_RCU.

You are the expert on that question, so your choice.  On this one,
I am but asking the questions.  ;-)

							Thanx, Paul

> Thanks.
> 
> > 
> > > ---
> > >  kernel/rcu/tree_exp.h | 10 ++++++++++
> > >  1 file changed, 10 insertions(+)
> > > 
> > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > > index 6058a734090c..87a44423927d 100644
> > > --- a/kernel/rcu/tree_exp.h
> > > +++ b/kernel/rcu/tree_exp.h
> > > @@ -406,8 +406,18 @@ static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp)
> > >  	for_each_leaf_node_cpu_mask(rnp, cpu, mask_ofl_ipi) {
> > >  		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> > >  		unsigned long mask = rdp->grpmask;
> > > +		int nr_retries = 0;
> > >  
> > >  retry_ipi:
> > > +		/*
> > > +		 * In case of retrying, CPU either is lagging:
> > > +		 *
> > > +		 * - between CPUHP_TEARDOWN_CPU and rcutree_report_cpu_dead()
> > > +		 * or:
> > > +		 * - between rcutree_report_cpu_starting() and set_cpu_online()
> > > +		 */
> > > +		WARN_ON_ONCE(nr_retries++ > 10);
> > > +
> > >  		if (rcu_watching_snap_stopped_since(rdp, rdp->exp_watching_snap)) {
> > >  			mask_ofl_test |= mask;
> > >  			continue;
> > > -- 
> > > 2.48.1
> > >