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

[Mukesh Ojha <quic_mojha@quicinc.com>]

On 2/10/2022 3:36 AM, Paul E. McKenney wrote:
> On Wed, Feb 09, 2022 at 11:53:33PM +0530, Mukesh Ojha wrote:
>> 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@quicinc.com>
>>> Cc: Tejun Heo <tj@kernel.org>
>>> Signed-off-by: Paul E. McKenney <paulmck@kernel.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
>> ?
> Hello, Mukesh, and thank you for looking this over!
>
> At first glance, I do not believe that this can happen because the
> expedited grace-period machinery avoids waiting on the current CPU.
> (See sync_rcu_exp_select_node_cpus(), both the raw_smp_processor_id()
> early in the function and the get_cpu() later in the function.)
>
> But please let me know if I am missing something here.
>
> But suppose that we could in fact reach this condition.  What bad thing
> would happen?  Other than a resched_cpu() having been invoked several
> times on a not-yet-online CPU, of course.  ;-)


I thought more about this, what if  synchronize_rcu_expedited thread got 
schedule out and run on some other cpu
and we clear out cpu on which it ran next from exp_mask.

Queuing the work on same cpu ensures that it will always be right cpu to 
clear out.
Do you think this can happen ?

-Mukesh

>
> 							Thanx, Paul