Hi Peter and Tglx,
We are observing another deadlock issue due to commit
0b26351b91(stop_machine, sched: Fix migrate_swap() vs. active_balance()
deadlock), even after taking the following fix
https://www.mail-archive.com/[email protected]/msg1740526.html
on the Linux-4.14.56 kernel.
Here is the scenario that leads to this deadlock.
We have used the stress-ng-64 --affinity test case to reproduce this
issue in a controlled environment, while simultaneously running CPU hot
plug and task migrations.
Stress-ng-affin (call stack shown below) is changing its own affinity
from cpu3 to cpu7. Stress-ng-affin is preempted in the
cpu_stop_queue_work() function
as soon as the stopper lock for migration/3 is released . At the same
time, on CPU 7, cross migration of tasks happens between cpu3 and cpu7.
=======================================================
Process: stress-ng-affin, cpu: 3 pid: 1748 start: 0xffffffd8817e4480
=====================================================
Task name: stress-ng-affin pid: 1748 cpu: 3 start: ffffffd8817e4480
state: 0x0 exit_state: 0x0 stack base: 0xffffff801c8e8000 Prio: 120
Stack:
[<ffffff87754864f4>] __switch_to+0xb8
[<ffffff87763ebf8c>] __schedule+0x690
[<ffffff87763ec388>] preempt_schedule_common+0x100
[<ffffff87763eb8f4>] preempt_schedule+0x24
[<ffffff87763f0e58>] _raw_spin_unlock_irqrestore+0x64
[<ffffff8775574f8c>] cpu_stop_queue_work+0x9c
[<ffffff8775574dfc>] stop_one_cpu+0x58
[<ffffff87754e4884>] __set_cpus_allowed_ptr+0x234
[<ffffff87754e8888>] sched_setaffinity+0x150
[<ffffff87754e8ad8>] SyS_sched_setaffinity+0xcc
[<ffffff87754837c0>] el0_svc_naked+0x34
[<0>] UNKNOWN+0x0
Due to cross migration of tasks between cpu7 and cpu3, migration/7 has
started executing and waits for the migration/3 task, so that they can
proceed within the multi cpu stop state machine together.
Unfortunately stress-ng-affin is affine to cpu7, and since migration 7
has started running, and has monopolized cpu7’s execution, stress-ng
will never run on cpu7, and cpu3’s migration task is never woken up.
Essentially:
Due to the nature of the wake_q interface, a thread can only be in at
most one wake queue at a time.
migration/3 is currently in stress-ng-affin’s wake_q. This means that no
other thread can add migration/3 to their wake queue.
Thus, even if any attempt is made to stop CPU 3 (e.g. cross-migration,
hot plugging, etc), no thread will wake up migration/3.
Below change helped to fix this deadlock.
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index e190d1e..f932e1e 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -87,9 +87,9 @@ static bool cpu_stop_queue_work(unsigned int cpu,
struct cpu_stop_work *work)
__cpu_stop_queue_work(stopper, work, &wakeq);
else if (work->done)
cpu_stop_signal_done(work->done);
- raw_spin_unlock_irqrestore(&stopper->lock, flags);
wake_up_q(&wakeq);
+ raw_spin_unlock_irqrestore(&stopper->lock, flags);
-Thanks, Prasad
--
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
Forum,
Linux Foundation Collaborative Project
On Wed, Aug 01, 2018 at 06:34:40PM -0700, Sodagudi Prasad wrote:
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index e190d1e..f932e1e 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -87,9 +87,9 @@ static bool cpu_stop_queue_work(unsigned int cpu, struct
> cpu_stop_work *work)
> __cpu_stop_queue_work(stopper, work, &wakeq);
> else if (work->done)
> cpu_stop_signal_done(work->done);
> - raw_spin_unlock_irqrestore(&stopper->lock, flags);
>
> wake_up_q(&wakeq);
> + raw_spin_unlock_irqrestore(&stopper->lock, flags);
>
That puts the wakeup back under stopper lock, which causes another
deadlock iirc.
On Thu, 2018-08-02 at 10:12 +0200, Peter Zijlstra wrote:
> On Wed, Aug 01, 2018 at 06:34:40PM -0700, Sodagudi Prasad wrote:
> > diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> > index e190d1e..f932e1e 100644
> > --- a/kernel/stop_machine.c
> > +++ b/kernel/stop_machine.c
> > @@ -87,9 +87,9 @@ static bool cpu_stop_queue_work(unsigned int cpu, struct
> > cpu_stop_work *work)
> > __cpu_stop_queue_work(stopper, work, &wakeq);
> > else if (work->done)
> > cpu_stop_signal_done(work->done);
> > - raw_spin_unlock_irqrestore(&stopper->lock, flags);
> >
> > wake_up_q(&wakeq);
> > + raw_spin_unlock_irqrestore(&stopper->lock, flags);
> >
>
> That puts the wakeup back under stopper lock, which causes another
> deadlock iirc.
Yup, one you fixed.
0b26351b910fb (Peter Zijlstra 2018-04-20 11:50:05 +0200 92) wake_up_q(&wakeq);
On Wed, Aug 01, 2018 at 06:34:40PM -0700, Sodagudi Prasad wrote:
> the Linux-4.14.56 kernel.
Can you also please run on something recent...
On Wed, Aug 01, 2018 at 06:34:40PM -0700, Sodagudi Prasad wrote:
> Due to cross migration of tasks between cpu7 and cpu3, migration/7 has
> started executing and waits for the migration/3 task, so that they can
> proceed within the multi cpu stop state machine together.
> Unfortunately stress-ng-affin is affine to cpu7, and since migration 7 has
> started running, and has monopolized cpu7’s execution, stress-ng will never
> run on cpu7, and cpu3’s migration task is never woken up.
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index e190d1e..f932e1e 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -87,9 +87,9 @@ static bool cpu_stop_queue_work(unsigned int cpu, struct
> cpu_stop_work *work)
> __cpu_stop_queue_work(stopper, work, &wakeq);
> else if (work->done)
> cpu_stop_signal_done(work->done);
> - raw_spin_unlock_irqrestore(&stopper->lock, flags);
>
> wake_up_q(&wakeq);
> + raw_spin_unlock_irqrestore(&stopper->lock, flags);
>
So why didn't you do the 'obvious' parallel to what you did for
cpu_stop_queue_two_works(), namely:
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -81,6 +81,7 @@ static bool cpu_stop_queue_work(unsigned
unsigned long flags;
bool enabled;
+ preempt_disable();
raw_spin_lock_irqsave(&stopper->lock, flags);
enabled = stopper->enabled;
if (enabled)
@@ -90,6 +91,7 @@ static bool cpu_stop_queue_work(unsigned
raw_spin_unlock_irqrestore(&stopper->lock, flags);
wake_up_q(&wakeq);
+ preempt_enable();
return enabled;
}
Prasad.
On Thu, 2 Aug 2018, Peter Zijlstra wrote:
>
> So why didn't you do the 'obvious' parallel to what you did for
> cpu_stop_queue_two_works(), namely:
Is that patch fixing the issue for you?
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -81,6 +81,7 @@ static bool cpu_stop_queue_work(unsigned
> unsigned long flags;
> bool enabled;
>
> + preempt_disable();
> raw_spin_lock_irqsave(&stopper->lock, flags);
> enabled = stopper->enabled;
> if (enabled)
> @@ -90,6 +91,7 @@ static bool cpu_stop_queue_work(unsigned
> raw_spin_unlock_irqrestore(&stopper->lock, flags);
>
> wake_up_q(&wakeq);
> + preempt_enable();
>
> return enabled;
> }
>
On 2018-08-03 04:41, Thomas Gleixner wrote:
> Prasad.
>
> On Thu, 2 Aug 2018, Peter Zijlstra wrote:
>>
>> So why didn't you do the 'obvious' parallel to what you did for
>> cpu_stop_queue_two_works(), namely:
>
> Is that patch fixing the issue for you?
<Prasad> Hi Thomas and Peter,
Yes. Tested both versions of patches and both variants are working on
Qualcomm devices
with stress testing of set affinity and tasks cross-migration, which
were previously leading to the deadlock.
-Thanks, Prasad
>
>> --- a/kernel/stop_machine.c
>> +++ b/kernel/stop_machine.c
>> @@ -81,6 +81,7 @@ static bool cpu_stop_queue_work(unsigned
>> unsigned long flags;
>> bool enabled;
>>
>> + preempt_disable();
>> raw_spin_lock_irqsave(&stopper->lock, flags);
>> enabled = stopper->enabled;
>> if (enabled)
>> @@ -90,6 +91,7 @@ static bool cpu_stop_queue_work(unsigned
>> raw_spin_unlock_irqrestore(&stopper->lock, flags);
>>
>> wake_up_q(&wakeq);
>> + preempt_enable();
>>
>> return enabled;
>> }
>>
--
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
Forum,
Linux Foundation Collaborative Project