Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup

["Kohli, Gaurav" <gkohli@codeaurora.org>]

On 5/1/2018 3:48 PM, Peter Zijlstra wrote:
> On Tue, May 01, 2018 at 01:20:26PM +0530, Kohli, Gaurav wrote:
>> But In our older case, where we have seen failure below is the wake up path
>> and ftraces, Wakeup occured  and completed before schedule call only.
>>
>> So final state of CPUHP is running not parked. I have also pasted debug
>> ftraces that we got during issue reproduction.
>>
>> Here wakeup for cpuhp is below:
>>
>> takedown_cpu-> kthread_park-> wake_up_process
>>
>>
>>   39,034,311,742,395  apps (10240)        Trace Printk cpuhp/0  (16)  [000]
>> 39015.625000: <debug> __kthread_parkme state=512 task=ffffffcc7458e680
>> flags: 0x5 -> state 5 -> state is parked inside parkme function
>>
>> 39,034,311,846,510  apps (10240)        Trace Printk cpuhp/0  (16)  [000]
>> 39015.625000: <debug> before schedule __kthread_parkme state=0
>> task=ffffffcc7458e680 flags: 0xd  ->  just before schedule call, state is
>> running
>>
>> tatic void __kthread_parkme(struct kthread *self)
>>
>> {
>>
>>          __set_current_state(TASK_PARKED);
>>
>>          while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
>>
>>                  if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags))
>>
>>                          complete(&self->parked);
>>
>>                  schedule();
>>
>>                  __set_current_state(TASK_PARKED);
>>
>>          }
>>
>>          clear_bit(KTHREAD_IS_PARKED, &self->flags);
>>
>>          __set_current_state(TASK_RUNNING);
>>
>> }
>>
>> So my point is here also, if it is reschedule then it can set TASK_PARKED,
>> but it seems after takedown_cpu call this thread never get a chance to run,
>> So final state is TASK_RUNNING.
>>
>> In our current fix also can't we observe same scenario where final state is
>> TASK_RUNNING.
> 
> I'm not sure I understand your concern. Loosing the TASK_PARKED store
> with the above code is obviously bad. But with the loop as proposed I
> don't see a problem.

Yes with loop, it will reset TASK_PARKED but that is not happening in 
the dumps we have seen. Here before schedule state is RUNNING and cpuhp
got migrate to some core but never get a chance to run so state is running.
> 
> takedown_cpu() can proceed beyond smpboot_park_threads() and kill the
> CPU before any of the threads are parked -- per having the complete()
> before hitting schedule().
> 
> And, afaict, that is harmless. When we go offline, sched_cpu_dying() ->
> migrate_tasks() will migrate any still runnable threads off the cpu.
> But because at this point the thread must be in the PARKED wait-loop, it
> will hit schedule() and go to sleep eventually.
> 
> Also note that kthread_unpark() does __kthread_bind() to rebind the
> threads.
> 
> Aaaah... I think I've spotted a problem there. We clear SHOULD_PARK
> before we rebind, so if the thread lost the first PARKED store,
> does the completion, gets migrated, cycles through the loop and now
> observes !SHOULD_PARK and bails the wait-loop, then __kthread_bind()
> will forever wait.
> 

So during next unpark
__kthread_unpark -> __kthread_bind -> wait_task_inactive (this got 
failed, as current state is running so failed on below call:


  while (task_running(rq, p)) {
                         if (match_state && unlikely(p->state != 
match_state))
                                 return 0;
                         cpu_relax();
                 }

and gives warning:


         if (!wait_task_inactive(p, state)) {
                 WARN_ON(1);
                 return; -> return from here, and further binding call 
fail which is after this code.
         }


finally it is giving bug_on here as we failed to rebind hotplug to our core:
                         }
                         kthread_parkme();
                         /* We might have been woken for stop */
                         continue;
                 }

                 BUG_ON(td->cpu != smp_processor_id()); panic occured.


So it seems we always have to be in PARKED state only , not miss any 
single instance.
> Is that what you had in mind?
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