Re: Large latency on blk_queue_enter

Jens Axboe
2017-05-08T15:40:39Z

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.

--
Jens Axboe