[PATCH] nvme: add a module parameter to change queue depth

[m.mizuma@jp.fujitsu.com (Masayoshi Mizuma)]

  
On Mon, 4 Jul 2016 01:49:40 -0700 Christoph Hellwig wrote:
> On Mon, Jul 04, 2016@05:33:20PM +0900, Masayoshi Mizuma wrote:
>> This patch adds "q_depth_limit" as a module parameter. nvme_queue->q_depth
>> is set below q_depth_limit.
>>
>> while loop at __nvme_process_cq() sometimes takes long time and
>> the loop is under IRQ context, so system slow down and hardlockup
>> may occur because of the loop.
>>
>> The while loop runs nvme_queue->q_depth times and the q_depth is set
>> by the following (NVME_Q_DEPTH is 1024).
>>
>>   dev->q_depth = min_t(int, NVME_CAP_MQES(cap) + 1, NVME_Q_DEPTH);
>>
>> To reduce the times of the loop, the q_depth_limit is useful.
>>
>> In addition, this patch moves the temporary fix for the Apple controller
>> into the new function, get_q_depth().
>
> While limiting the queue depth might still be useful I think we need
> to look into a potential lock break in __nvme_process_cq so that it
> doesn't cause hard lockups even with large queue depth first.

Yes, the module parameter is useful as a workaround to avoid hard lockup.

I agree that we should look into nvme_queue->q_lock lock break
in __nvme_process_cq().

However, if __nvme_process_cq() is called by nvme_irq() in IRQ context,
the hard lockup may happen even if nvme_queue->q_lock is unlocked.
This is because timer interrupt is disabled while the IRQ context.

- Masayoshi Mizuma

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