Re: Large latency on blk_queue_enter

Javier González
2017-05-08T14:46:40Z

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> 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.

[1] https://github.com/OpenChannelSSD/liblightnvm

Thanks!