Re: Large latency on blk_queue_enter

Javier González
2017-05-08T15:38:47Z

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> 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 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.