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

[Frederic Weisbecker]

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@xxxxxxxxxx>
> > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> 
> Same CONFIG_PROVE_RCU question, same conditional:
> 
> Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

I don't have a strong opinion whether to keep this warning unconditional.
Perhaps this can depend on CONFIG_PROVE_RCU.

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
> >