Re: Large latency on blk_queue_enter

Jens Axboe
2017-05-08T14:52:54Z

On 05/08/2017 08:46 AM, Javier González wrote:
>> On 8 May 2017, at 16.23, Jens Axboe <axboe@fb.com> wrote:
>>
>> On 05/08/2017 08:20 AM, Javier González wrote:
>>>> 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...
>>
>> How do I reproduce this? Off the top of my head, and looking at the code,
>> I have no idea what is going on here.
>
> Using LightNVM and liblightnvm [1] you can reproduce it by:
>
> 1. Instantiate a pblk instance on the first channel (luns 0 - 7):
> sudo nvme lnvm create -d nvme0n1 -n test0 -t pblk -b 0 -e 7 -f
> 2. Write 5GB to the test0 block device with a normal fio script
> 3. Read 5GB to verify that latencies are good (max. ~80-90us at bs=4k, qd=1)
> 4. Re-run 3. and in parallel send a command through liblightnvm to a
> different channel. A simple command is an erase (erase block 900 on
> channel 2, lun 0):
> sudo nvm_vblk line_erase /dev/nvme0n1 2 2 0 0 900
>
> After 4. you should see a ~25-30ms latency on the read workload.
>
> I tried to reproduce the ioctl in a more generic way to reach
> __nvme_submit_user_cmd(), but SPDK steals the whole device. Also, qemu
> is not reliable for this kind of performance testing.
>
> If you have a suggestion on how I can mix an ioctl with normal block I/O
> read on a standard NVMe device, I'm happy to try it and see if I can
> reproduce the issue.

Just to rule out this being any hardware related delays in processing
IO:

1) Does it reproduce with a simpler command, anything close to a no-op
that you can test?
2) What did you use to time the stall being blk_queue_enter()?

--
Jens Axboe