* [PATCH rcu 0/3] Expedited-grace-period updates for v5.18
@ 2022-02-04 22:54 Paul E. McKenney
2022-02-04 22:55 ` [PATCH rcu 1/3] rcu/exp: Fix check for idle context in rcu_exp_handler Paul E. McKenney
` (2 more replies)
0 siblings, 3 replies; 12+ messages in thread
From: Paul E. McKenney @ 2022-02-04 22:54 UTC (permalink / raw)
To: rcu; +Cc: linux-kernel, kernel-team, rostedt
Hello!
This series provides updates for RCU expedited grace periods:
1. Fix check for idle context in rcu_exp_handler, courtesy of
Neeraj Upadhyay.
2. Mark ->expmask access in synchronize_rcu_expedited_wait().
3. Allow expedited RCU grace periods on incoming CPUs.
Thanx, Paul
------------------------------------------------------------------------
b/kernel/rcu/tree_exp.h | 2 +-
kernel/rcu/tree_exp.h | 17 ++++++++++++-----
2 files changed, 13 insertions(+), 6 deletions(-)
^ permalink raw reply [flat|nested] 12+ messages in thread
* [PATCH rcu 1/3] rcu/exp: Fix check for idle context in rcu_exp_handler
2022-02-04 22:54 [PATCH rcu 0/3] Expedited-grace-period updates for v5.18 Paul E. McKenney
@ 2022-02-04 22:55 ` Paul E. McKenney
2022-02-04 22:55 ` [PATCH rcu 2/3] rcu: Mark ->expmask access in synchronize_rcu_expedited_wait() Paul E. McKenney
2022-02-04 22:55 ` [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs Paul E. McKenney
2 siblings, 0 replies; 12+ messages in thread
From: Paul E. McKenney @ 2022-02-04 22:55 UTC (permalink / raw)
To: rcu
Cc: linux-kernel, kernel-team, rostedt, Neeraj Upadhyay,
Frederic Weisbecker, Paul E . McKenney
From: Neeraj Upadhyay <quic_neeraju@quicinc.com>
For PREEMPT_RCU, the rcu_exp_handler() function checks
whether the current CPU is in idle, by calling
rcu_dynticks_curr_cpu_in_eqs(). However, rcu_exp_handler()
is called in IPI handler context. So, it should be checking
the idle context using rcu_is_cpu_rrupt_from_idle(). Fix this
by using rcu_is_cpu_rrupt_from_idle() instead of
rcu_dynticks_curr_cpu_in_eqs(). Non-preempt configuration
already uses the correct check.
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/tree_exp.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 237a79989abae..1568c8ef185b2 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -656,7 +656,7 @@ static void rcu_exp_handler(void *unused)
*/
if (!depth) {
if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
- rcu_dynticks_curr_cpu_in_eqs()) {
+ rcu_is_cpu_rrupt_from_idle()) {
rcu_report_exp_rdp(rdp);
} else {
WRITE_ONCE(rdp->cpu_no_qs.b.exp, true);
--
2.31.1.189.g2e36527f23
^ permalink raw reply related [flat|nested] 12+ messages in thread
* [PATCH rcu 2/3] rcu: Mark ->expmask access in synchronize_rcu_expedited_wait()
2022-02-04 22:54 [PATCH rcu 0/3] Expedited-grace-period updates for v5.18 Paul E. McKenney
2022-02-04 22:55 ` [PATCH rcu 1/3] rcu/exp: Fix check for idle context in rcu_exp_handler Paul E. McKenney
@ 2022-02-04 22:55 ` Paul E. McKenney
2022-02-04 22:55 ` [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs Paul E. McKenney
2 siblings, 0 replies; 12+ messages in thread
From: Paul E. McKenney @ 2022-02-04 22:55 UTC (permalink / raw)
To: rcu; +Cc: linux-kernel, kernel-team, rostedt, Paul E. McKenney
This commit adds a READ_ONCE() to an access to the rcu_node structure's
->expmask field to prevent compiler mischief. Detected by KCSAN.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/tree_exp.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 1568c8ef185b2..60197ea24ceb9 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -502,7 +502,8 @@ static void synchronize_rcu_expedited_wait(void)
if (synchronize_rcu_expedited_wait_once(1))
return;
rcu_for_each_leaf_node(rnp) {
- for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {
+ mask = READ_ONCE(rnp->expmask);
+ for_each_leaf_node_cpu_mask(rnp, cpu, mask) {
rdp = per_cpu_ptr(&rcu_data, cpu);
if (rdp->rcu_forced_tick_exp)
continue;
--
2.31.1.189.g2e36527f23
^ permalink raw reply related [flat|nested] 12+ messages in thread
* [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs
2022-02-04 22:54 [PATCH rcu 0/3] Expedited-grace-period updates for v5.18 Paul E. McKenney
2022-02-04 22:55 ` [PATCH rcu 1/3] rcu/exp: Fix check for idle context in rcu_exp_handler Paul E. McKenney
2022-02-04 22:55 ` [PATCH rcu 2/3] rcu: Mark ->expmask access in synchronize_rcu_expedited_wait() Paul E. McKenney
@ 2022-02-04 22:55 ` Paul E. McKenney
2022-02-08 18:56 ` Tejun Heo
2022-02-09 18:23 ` Mukesh Ojha
2 siblings, 2 replies; 12+ messages in thread
From: Paul E. McKenney @ 2022-02-04 22:55 UTC (permalink / raw)
To: rcu
Cc: linux-kernel, kernel-team, rostedt, Paul E. McKenney,
Mukesh Ojha, Tejun Heo
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);
--
2.31.1.189.g2e36527f23
^ permalink raw reply related [flat|nested] 12+ messages in thread
* Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs
2022-02-04 22:55 ` [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs Paul E. McKenney
@ 2022-02-08 18:56 ` Tejun Heo
2022-02-09 18:23 ` Mukesh Ojha
1 sibling, 0 replies; 12+ messages in thread
From: Tejun Heo @ 2022-02-08 18:56 UTC (permalink / raw)
To: Paul E. McKenney; +Cc: rcu, linux-kernel, kernel-team, rostedt, Mukesh Ojha
On Fri, Feb 04, 2022 at 02:55:07PM -0800, 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>
Acked-by: Tejun Heo <tj@kernel.org>
Thanks.
--
tejun
^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs
2022-02-04 22:55 ` [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs Paul E. McKenney
2022-02-08 18:56 ` Tejun Heo
@ 2022-02-09 18:23 ` Mukesh Ojha
2022-02-09 22:06 ` Paul E. McKenney
1 sibling, 1 reply; 12+ messages in thread
From: Mukesh Ojha @ 2022-02-09 18:23 UTC (permalink / raw)
To: Paul E. McKenney, rcu; +Cc: linux-kernel, kernel-team, rostedt, Tejun Heo
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 ?
-Mukesh
^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs
2022-02-09 18:23 ` Mukesh Ojha
@ 2022-02-09 22:06 ` Paul E. McKenney
2022-02-11 18:44 ` Mukesh Ojha
0 siblings, 1 reply; 12+ messages in thread
From: Paul E. McKenney @ 2022-02-09 22:06 UTC (permalink / raw)
To: Mukesh Ojha; +Cc: rcu, linux-kernel, kernel-team, rostedt, Tejun Heo
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. ;-)
Thanx, Paul
^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs
2022-02-09 22:06 ` Paul E. McKenney
@ 2022-02-11 18:44 ` Mukesh Ojha
2022-02-11 22:14 ` Paul E. McKenney
0 siblings, 1 reply; 12+ messages in thread
From: Mukesh Ojha @ 2022-02-11 18:44 UTC (permalink / raw)
To: paulmck; +Cc: rcu, linux-kernel, kernel-team, rostedt, Tejun Heo
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
^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs
2022-02-11 18:44 ` Mukesh Ojha
@ 2022-02-11 22:14 ` Paul E. McKenney
2022-02-12 8:47 ` Mukesh Ojha
0 siblings, 1 reply; 12+ messages in thread
From: Paul E. McKenney @ 2022-02-11 22:14 UTC (permalink / raw)
To: Mukesh Ojha; +Cc: rcu, linux-kernel, kernel-team, rostedt, Tejun Heo
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?
Thanx, Paul
^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs
2022-02-11 22:14 ` Paul E. McKenney
@ 2022-02-12 8:47 ` Mukesh Ojha
2022-02-12 11:28 ` Neeraj Upadhyay
0 siblings, 1 reply; 12+ messages in thread
From: Mukesh Ojha @ 2022-02-12 8:47 UTC (permalink / raw)
To: paulmck; +Cc: rcu, linux-kernel, kernel-team, rostedt, Tejun Heo
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.
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
^ permalink raw reply related [flat|nested] 12+ messages in thread
* Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs
2022-02-12 8:47 ` Mukesh Ojha
@ 2022-02-12 11:28 ` Neeraj Upadhyay
2022-02-12 13:56 ` Mukesh Ojha
0 siblings, 1 reply; 12+ messages in thread
From: Neeraj Upadhyay @ 2022-02-12 11:28 UTC (permalink / raw)
To: Mukesh Ojha, paulmck; +Cc: rcu, linux-kernel, kernel-team, rostedt, Tejun Heo
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
[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);
^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs
2022-02-12 11:28 ` Neeraj Upadhyay
@ 2022-02-12 13:56 ` Mukesh Ojha
0 siblings, 0 replies; 12+ messages in thread
From: Mukesh Ojha @ 2022-02-12 13:56 UTC (permalink / raw)
To: Neeraj Upadhyay, paulmck
Cc: rcu, linux-kernel, kernel-team, rostedt, Tejun Heo
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);
^ permalink raw reply [flat|nested] 12+ messages in thread
end of thread, other threads:[~2022-02-12 13:56 UTC | newest]
Thread overview: 12+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-02-04 22:54 [PATCH rcu 0/3] Expedited-grace-period updates for v5.18 Paul E. McKenney
2022-02-04 22:55 ` [PATCH rcu 1/3] rcu/exp: Fix check for idle context in rcu_exp_handler Paul E. McKenney
2022-02-04 22:55 ` [PATCH rcu 2/3] rcu: Mark ->expmask access in synchronize_rcu_expedited_wait() Paul E. McKenney
2022-02-04 22:55 ` [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs Paul E. McKenney
2022-02-08 18:56 ` Tejun Heo
2022-02-09 18:23 ` Mukesh Ojha
2022-02-09 22:06 ` Paul E. McKenney
2022-02-11 18:44 ` Mukesh Ojha
2022-02-11 22:14 ` Paul E. McKenney
2022-02-12 8:47 ` Mukesh Ojha
2022-02-12 11:28 ` Neeraj Upadhyay
2022-02-12 13:56 ` Mukesh Ojha
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