linux-kernel - Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs

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Message-ID: <de3a9710-fbf4-8005-a781-adc95ae4a090@quicinc.com>
Date:   Wed, 9 Feb 2022 23:53:33 +0530
From:   Mukesh Ojha <quic_mojha@...cinc.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>, <rcu@...r.kernel.org>
CC:     <linux-kernel@...r.kernel.org>, <kernel-team@...com>,
        <rostedt@...dmis.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on
 incoming CPUs


On 2/5/2022 4:25 AM, Paul E. McKenney wrote:
> Although it is usually safe to invoke synchronize_rcu_expedited() from a
> preemption-enabled CPU-hotplug notifier, if it is invoked from a notifier
> between CPUHP_AP_RCUTREE_ONLINE and CPUHP_AP_ACTIVE, its attempts to
> invoke a workqueue handler will hang due to RCU waiting on a CPU that
> the scheduler is not paying attention to.  This commit therefore expands
> use of the existing workqueue-independent synchronize_rcu_expedited()
> from early boot to also include CPUs that are being hotplugged.
>
> Link: https://lore.kernel.org/lkml/7359f994-8aaf-3cea-f5cf-c0d3929689d6@quicinc.com/
> Reported-by: Mukesh Ojha <quic_mojha@...cinc.com>
> Cc: Tejun Heo <tj@...nel.org>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> ---
>   kernel/rcu/tree_exp.h | 14 ++++++++++----
>   1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 60197ea24ceb9..1a45667402260 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -816,7 +816,7 @@ static int rcu_print_task_exp_stall(struct rcu_node *rnp)
>    */
>   void synchronize_rcu_expedited(void)
>   {
> -	bool boottime = (rcu_scheduler_active == RCU_SCHEDULER_INIT);
> +	bool no_wq;
>   	struct rcu_exp_work rew;
>   	struct rcu_node *rnp;
>   	unsigned long s;
> @@ -841,9 +841,15 @@ void synchronize_rcu_expedited(void)
>   	if (exp_funnel_lock(s))
>   		return;  /* Someone else did our work for us. */
>   
> +	/* Don't use workqueue during boot or from an incoming CPU. */
> +	preempt_disable();
> +	no_wq = rcu_scheduler_active == RCU_SCHEDULER_INIT ||
> +		!cpumask_test_cpu(smp_processor_id(), cpu_active_mask);
> +	preempt_enable();
> +
>   	/* Ensure that load happens before action based on it. */
> -	if (unlikely(boottime)) {
> -		/* Direct call during scheduler init and early_initcalls(). */
> +	if (unlikely(no_wq)) {
> +		/* Direct call for scheduler init, early_initcall()s, and incoming CPUs. */
>   		rcu_exp_sel_wait_wake(s);
>   	} else {
>   		/* Marshall arguments & schedule the expedited grace period. */
> @@ -861,7 +867,7 @@ void synchronize_rcu_expedited(void)
>   	/* Let the next expedited grace period start. */
>   	mutex_unlock(&rcu_state.exp_mutex);
>   
> -	if (likely(!boottime))
> +	if (likely(!no_wq))
>   		destroy_work_on_stack(&rew.rew_work);
>   }
>   EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);

Can we reach a condition after this change where no_wq = true and during 
rcu_stall report where exp_task = 0 list and exp_mask contain only this 
cpu ?

-Mukesh