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Message-ID: <d0118f40-399b-5b50-f730-1ead42eff364@quicinc.com>
Date: Sat, 12 Feb 2022 19:26:08 +0530
From: Mukesh Ojha <quic_mojha@...cinc.com>
To: Neeraj Upadhyay <quic_neeraju@...cinc.com>, <paulmck@...nel.org>
CC: <rcu@...r.kernel.org>, <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 2/12/2022 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@...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
>>>>>> ?
>>>>> 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);
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