Re: Large latency on blk_queue_enter

Javier González
2017-05-08T14:20:42Z

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> On 8 May 2017, at 16.13, Jens Axboe <axboe@fb.com> wrote:
>
> On 05/08/2017 07:44 AM, Javier González wrote:
>>> On 8 May 2017, at 14.27, Ming Lei <ming.lei@redhat.com> wrote:
>>>
>>> On Mon, May 08, 2017 at 01:54:58PM +0200, Javier González wrote:
>>>> Hi,
>>>>
>>>> I find an unusual added latency(~20-30ms) on blk_queue_enter when
>>>> allocating a request directly from the NVMe driver through
>>>> nvme_alloc_request. I could use some help confirming that this is a bug
>>>> and not an expected side effect due to something else.
>>>>
>>>> I can reproduce this latency consistently on LightNVM when mixing I/O
>>>> from pblk and I/O sent through an ioctl using liblightnvm, but I don't
>>>> see anything on the LightNVM side that could impact the request
>>>> allocation.
>>>>
>>>> When I have a 100% read workload sent from pblk, the max. latency is
>>>> constant throughout several runs at ~80us (which is normal for the media
>>>> we are using at bs=4k, qd=1). All pblk I/Os reach the nvme_nvm_submit_io
>>>> function on lightnvm.c., which uses nvme_alloc_request. When we send a
>>>> command from user space through an ioctl, then the max latency goes up
>>>> to ~20-30ms. This happens independently from the actual command
>>>> (IN/OUT). I tracked down the added latency down to the call
>>>> percpu_ref_tryget_live in blk_queue_enter. Seems that the queue
>>>> reference counter is not released as it should through blk_queue_exit in
>>>> blk_mq_alloc_request. For reference, all ioctl I/Os reach the
>>>> nvme_nvm_submit_user_cmd on lightnvm.c
>>>>
>>>> Do you have any idea about why this might happen? I can dig more into
>>>> it, but first I wanted to make sure that I am not missing any obvious
>>>> assumption, which would explain the reference counter to be held for a
>>>> longer time.
>>>
>>> You need to check if the .q_usage_counter is working at atomic mode.
>>> This counter is initialized as atomic mode, and finally switchs to
>>> percpu mode via percpu_ref_switch_to_percpu() in blk_register_queue().
>>
>> Thanks for commenting Ming.
>>
>> The .q_usage_counter is not working on atomic mode. The queue is
>> initialized normally through blk_register_queue() and the counter is
>> switched to percpu mode, as you mentioned. As I understand it, this is
>> how it should be, right?
>
> That is how it should be, yes. You're not running with any heavy
> debugging options, like lockdep or anything like that?

No lockdep, KASAN, kmemleak or any of the other usual suspects.

What's interesting is that it only happens when one of the I/Os comes
from user space through the ioctl. If I have several pblk instances on
the same device (which would end up allocating a new request in
parallel, potentially on the same core), the latency spike does not
trigger.

I also tried to bind the read thread and the liblightnvm thread issuing
the ioctl to different cores, but it does not help...