lkml.org 
[lkml]   [2022]   [Feb]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs
From

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

\
 
 \ /
  Last update: 2022-02-11 19:46    [W:0.057 / U:0.120 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site