[RFC PATCH] nvme: always return IRQ_HANDLED

[Jens Axboe]

On 08/17/2017 02:29 PM, Keith Busch wrote:
> On Thu, Aug 17, 2017 at 02:17:08PM -0600, Jens Axboe wrote:
>> On 08/17/2017 02:15 PM, Keith Busch wrote:
>>> On Thu, Aug 17, 2017 at 01:32:20PM -0600, Jens Axboe wrote:
>>>> We currently have an issue with nvme when polling is used. Just
>>>> ran some testing on 4.13-rc5, and it's trivial to trigger an IRQ
>>>> disable ala:
>>>>
>>>> [   52.412851] irq 77: nobody cared (try booting with the "irqpoll" option)
>>>> [   52.415310] irq 70: nobody cared (try booting with the "irqpoll" option)
>>>>
>>>> when running a few processes polling. The reason is pretty obvious - if
>>>> we're effective at polling, the triggered IRQ will never find any
>>>> events. If this happens enough times in a row, the kernel disables our
>>>> IRQ since we keep returning IRQ_NONE.
>>>
>>> If you're seeing IRQ_NONE returned, the NVMe driver didn't poll any
>>> completions since the last time nvme_irq was called. The cqe_seen on
>>> polled compeletions is sticky until the IRQ handler is run, in which
>>> case it returns IRQ_HANDLED even when no completions were handled during
>>> that interrupt.
>>>
>>> The only way it should be able to return IRQ_NONE is if no completions
>>> were observed (polled or otherwise) since the last time the IRQ handler
>>> was called.
>>
>> The polling do not update the cqe_seen. So it's possible that every time
>> the IRQ handler does trigger, there are no entries found. Maybe a better
>> or simpler fix would be to have the polling set cqe_seen to true, and
>> leave the clearing to the interrupt handler as is done now.
> 
> Oops, that looks like a mistake. I don't think we want to suppress
> spurious interrupt detection, though. How about this patch to set cq_seen
> on polling like we used to have?

Yes, that will work. We need to get this into 4.13.

-- 
Jens Axboe