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Message-ID: <2a4870e9-41ae-60cf-fef3-aeb1de6f559c@quicinc.com>
Date: Sat, 12 Feb 2022 14:17:02 +0530
From: Mukesh Ojha <quic_mojha@...cinc.com>
To: <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 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.
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);
--
2.7.4
-Mukesh
> Thanx, Paul
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