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SubjectRe: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup
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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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Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project.

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