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Message-ID: <73bc88ae-a1a4-99e2-0e66-e2ce553b2f46@quicinc.com>
Date: Sat, 12 Feb 2022 16:58:54 +0530
From: Neeraj Upadhyay <quic_neeraju@...cinc.com>
To: Mukesh Ojha <quic_mojha@...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
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
[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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