Re: Large latency on blk_queue_enter

Javier González
2017-05-08T15:22:47Z

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Javier

> On 8 May 2017, at 17.14, Jens Axboe <axboe@kernel.dk> wrote:
>
> On 05/08/2017 09:08 AM, Jens Axboe wrote:
>> On 05/08/2017 09:02 AM, Javier González wrote:
>>>> 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.
>>
>> OK, that's starting to make more sense, that indicates that there is indeed
>> something wrong with the refs. Does the below help?
>
> No, that can't be right, it does look balanced to begin with.
> blk_mq_alloc_request() always grabs a queue ref, and always drops it. If
> we return with a request succesfully allocated, then we have an extra
> ref on it, which is dropped when it is later freed.

I agree, it seems more like a reference is put too late. I looked into
into the places where the reference is put, but it all seems normal. In
any case, I run it (just to see), and it did not help.

> Something smells fishy, I'll dig a bit.

Thanks! I continue looking into it myself; let me know if I can help
with something more specific.