Re: Large latency on blk_queue_enter

Javier González
2017-05-08T16:39:30Z

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> On 8 May 2017, at 18.06, Jens Axboe <axboe@kernel.dk> wrote:
>
> On 05/08/2017 09:49 AM, Javier González wrote:
>>> On 8 May 2017, at 17.40, Jens Axboe <axboe@kernel.dk> wrote:
>>>
>>> On 05/08/2017 09:38 AM, Javier González wrote:
>>>>> On 8 May 2017, at 17.25, Jens Axboe <axboe@kernel.dk> wrote:
>>>>>
>>>>> 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"?
>>>>
>>>> I can reproduce the issue on 4.11-rc7. I will rebase on top of your
>>>> for-4.12/block, but I cannot see any patches that might be related. If
>>>> it changes I'll ping you.
>>>
>>> I don't suspect it will do anything for you. I just ask to know what
>>> base you are on.
>>>
>>>> I mentioned the problem to Christoph last week and disabling the
>>>> schedulers was the first thing he recommended. I measured time around
>>>> blk_mq_sched_get_request and for this particular test the choose of
>>>> scheduler (including BFQ and kyber) does not seem to have an effect.
>>>
>>> kyber vs none would be the interesting test. Some of the paths are a
>>> little different depending if there's a scheduler attached or not, so
>>> it's good to know that we're seeing this in both cases.
>>
>> I just tested on your for-4.12/block with none and kyber and the latency
>> spike appears in both cases.
>
> OK good. I looked at your reproduction case. Looks like we ultimately
> end up submitting IO through nvme_nvm_submit_user_cmd() when you do the
> nvm_vblk line_erase, which is basically the same code as
> NVME_IOCTL_SUBMIT_IO as far as request alloc, setup, issue, free goes.
> So does it reproduce for you as well on a normal nvme device, if you run
> a nvme read /dev/nvme0 [...] while running the same read fio job?

Ok. I'll try that.

Thanks!