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

[Mukesh Ojha <quic_mojha@quicinc.com>]

On 2/12/2022 4:58 PM, Neeraj Upadhyay wrote:
> Hi Mukesh,
>
> On 2/12/2022 2:17 PM, Mukesh Ojha wrote:
>>
>> On 2/12/2022 3:44 AM, Paul E. McKenney wrote:
>>> On Sat, Feb 12, 2022 at 12:14:20AM +0530, Mukesh Ojha wrote:
>>>> 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 ?
>>> Indeed it might.
>>>
>>> But if it did, the scheduler would invoke RCU's hook, which is named
>>> rcu_note_context_switch(), and do so on the pre-switch CPU. There are
>>> two implementations for this function, one for CONFIG_PREEMPT=y
>>> and another for CONFIG_PREEMPT=n.  Both look to me like they invoke
>>> rcu_report_exp_rdp() when needed, one directly and the other via the
>>> CONFIG_PREEMPT=n variant of rcu_qs().
>>>
>>> Am I missing something?
>>>
>>>
>>
>> There is a issue we are facing where exp_mask is not getting cleared 
>> and rcu_stall report that
>> the cpu we are waiting on sometime in idle and sometime executing 
>> some other task but
>> it is not clearing itself from exp_mask from a very long time and in 
>> all the instances exp_task list is NULL.
>
> Can you please check whether [1] is present in your tree?
>
Thanks Neeraj.
It is not there, will check the results with this patch.

-Mukesh

>
>
> Thanks
> Neeraj
>
> [1] 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/kernel/rcu/tree_exp.h?h=v5.17-rc3&id=81f6d49cce2d2fe507e3fddcc4a6db021d9c2e7b
>>
>>     expmask = 8,     ==> cpu3
>>
>> [80235.522440][T12441] rcu: INFO: rcu_preempt detected expedited 
>> stalls on CPUs/tasks: { 3-... } 9163622 jiffies s: 634705 root: 0x8/.
>> [80235.534757][T12441] rcu: blocking rcu_node structures:
>> [80235.540102][T12441] Task dump for CPU 3:
>> [80235.540118][T12441] task:core_ctl        state:D stack:    0 pid: 
>> 172 ppid:     2 flags:0x00000008
>> [80235.540150][T12441] Call trace:
>> [80235.540178][T12441]  __switch_to+0x2a8/0x3ac
>> [80235.540207][T12441]  rcu_state+0x11b0/0x1480
>>
>>
>> [80299.010105][T12441] rcu: INFO: rcu_preempt detected expedited 
>> stalls on CPUs/tasks: { 3-... } 9179494 jiffies s: 634705 root: 0x8/.
>> [80299.022623][T12441] rcu: blocking rcu_node structures:
>> [80299.027924][T12441] Task dump for CPU 3:
>> [80299.027942][T12441] task:swapper/3       state:R  running task 
>> stack:    0 pid:    0 ppid:     1 flags:0x00000008
>> [80299.027993][T12441] Call trace:
>> [80299.028025][T12441]  __switch_to+0x2a8/0x3ac
>> [80299.028051][T12441]  0xffffffc010113eb4
>>
>>
>> As we were not seeing this earlier.
>> Below is compile tested patch, can we do something like this  ?
>>
>> ==========================================><==================================================== 
>>
>>
>> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
>> index 6453ac5..f0332e4 100644
>> --- a/kernel/rcu/tree_exp.h
>> +++ b/kernel/rcu/tree_exp.h
>> @@ -812,10 +812,12 @@ static int rcu_print_task_exp_stall(struct 
>> rcu_node *rnp)
>>    */
>>   void synchronize_rcu_expedited(void)
>>   {
>> -    bool no_wq;
>> +    bool no_wq = (rcu_scheduler_active == RCU_SCHEDULER_INIT);
>> +    bool is_active;
>>       struct rcu_exp_work rew;
>>       struct rcu_node *rnp;
>>       unsigned long s;
>> +    int next_cpu;
>>
>>       RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
>>                lock_is_held(&rcu_lock_map) ||
>> @@ -837,19 +839,28 @@ 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(no_wq)) {
>> -        /* Direct call during scheduler init, early_initcalls() and 
>> incoming CPUs. */
>> +        /* Direct call during scheduler init, early_initcalls(). */
>>           rcu_exp_sel_wait_wake(s);
>> +        mutex_unlock(&rcu_state.exp_mutex);
>> +        return;
>> +    }
>> +
>> +    preempt_disable();
>> +    is_active = cpumask_test_cpu(smp_processor_id(), cpu_active_mask);
>> +    preempt_enable();
>> +
>> +    rew.rew_s = s;
>> +    if (!is_active) {
>> +        INIT_WORK(&rew.rew_work, wait_rcu_exp_gp);
>> +        next_cpu = cpumask_next(smp_processor_id(), cpu_active_mask);
>> +        if (next_cpu >= nr_cpu_ids)
>> +            next_cpu = cpumask_first(cpu_active_mask);
>> +
>> +        queue_work_on(next_cpu, rcu_gp_wq, &rew.rew_work);
>>       } else {
>>           /* Marshall arguments & schedule the expedited grace 
>> period. */
>> -        rew.rew_s = s;
>>           INIT_WORK_ONSTACK(&rew.rew_work, wait_rcu_exp_gp);
>>           queue_work(rcu_gp_wq, &rew.rew_work);
>>       }
>> @@ -863,7 +874,9 @@ void synchronize_rcu_expedited(void)
>>       /* Let the next expedited grace period start. */
>>       mutex_unlock(&rcu_state.exp_mutex);
>>
>> -    if (likely(!no_wq))
>> +    if (likely(is_active))
>>           destroy_work_on_stack(&rew.rew_work);
>> +    else
>> +        flush_work(&rew.rew_work);
>>   }
>>   EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);