Re: Large latency on blk_queue_enter

Javier González
2017-05-08T15:02:53Z

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> On 8 May 2017, at 16.52, Jens Axboe <axboe@fb.com> wrote:
>
> 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?

Yes. I tried with a 4KB read and with a fake command I drop right after
allocation.

> 2) What did you use to time the stall being blk_queue_enter()?
>

I have some debug code measuring time with ktime_get() in different
places in the stack, and among other places, around blk_queue_enter(). I
use them then to measure max latency and expose it through sysfs. I can
see that the latency peak is recorded in the probe before
blk_queue_enter() and not in the one after.

I also did an experiment, where the normal I/O path allocates the
request with BLK_MQ_REQ_NOWAIT. When running the experiment above, the
read test fails since we reach:
if (nowait)
return -EBUSY;

in blk_queue_enter.