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

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

On 5/2/2018 1:50 PM, Peter Zijlstra wrote:
> On Wed, May 02, 2018 at 10:45:52AM +0530, Kohli, Gaurav wrote:
>> On 5/1/2018 6:49 PM, Peter Zijlstra wrote:
> 
>>>    - complete(&kthread->parked), which we can do inside schedule(); this
>>>      solves the problem because then kthread_park() will not return early
>>>      and the task really is blocked.
>>
>> I think complete will not help, as problem is like below :
>>
>> Control Thread                                CPUHP thread
>> 					
>> 					      cpuhp_thread_fun
>> 					      Wake control thread
>> 					      complete(&st->done);
>>
>> takedown_cpu
>> kthread_park
>> set_bit(KTHREAD_SHOULD_PARK
>>
>> 					     Here cpuhp is looping,
>> 					//success case
>> 					     Generally when issue is not
>> 					     coming
>> 					     it schedule out by below :
>>                                             ht->thread_should_run(td->cpu
>> 					      scheduler
>> 					//failure case
>> 					before schedule
>> 					loop check
>> 					(kthread_should_park()
>> 				         enter here as PARKED set
>>
>> wake_up_process(k)
> 
> If k has TASK_PARKED, then wake_up_process() which uses TASK_NORMAL will
> no-op, because:
> 
> 	TASK_PARKED & TASK_NORMAL == 0
> 
>> 					__kthread_parkme
>> 					 complete(&self->parked);
>> SETS RUNNING
>> 		                         schedule			
> 
> But suppose, you do get that store, and we get to schedule with
> TASK_RUNNING, then schedule will no-op and we'll go around the loop and
> not complete.
> 
> See also: lkml.kernel.org/r/20180430111744.GE4082@hirez.programming.kicks-ass.net
> 
> Either TASK_RUNNING gets set before we do schedule() and we go around
> again, re-set TASK_PARKED, resched the condition and re-call schedule(),
> or we schedule() first and ttwu() will not issue the TASK_RUNNING store.
> 
> In either case, we'll eventually hit schedule() with TASK_PARKED. Then,
> and only then will the complete() happen.
> 
>> wait_for_completion(&kthread->parked);
> 
> The point is, we'll only ever complete ^ that completion when we've
> scheduled out the task in TASK_PARKED state. If the task didn't get
> parked, no completion.

Thanks for the detailed explanation, yes in all cases unpark will 
observe parked state only.
> 
> 
> And that is the reason I like this approach above the others. It
> guarantees the task really is parked when we ask for it. We don't have
> to deal with the task still running and getting migrated to another CPU
> nonsense.
> 

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