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: <bf7a3b05-4218-42aa-b936-8fc86435db15@paulmck-laptop>
Date: Tue, 18 Mar 2025 10:22:33 -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 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>

> ---
>  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
>