Re: Large latency on blk_queue_enter

Jens Axboe
2017-05-08T15:25:44Z

On 05/08/2017 09:22 AM, Javier González wrote:
>
> 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.

What exact kernel are you running? And does the device have a scheduler
attached, or is it set to "none"?

--
Jens Axboe