Re: Large latency on blk_queue_enter

[Javier GonzÃlez]

> On 8 May 2017, at 18.39, Javier GonzÃlez <jg@xxxxxxxxxxx> wrote:
> 
>> On 8 May 2017, at 18.06, Jens Axboe <axboe@xxxxxxxxx> wrote:
>> 
>> On 05/08/2017 09:49 AM, Javier GonzÃlez wrote:
>>>> On 8 May 2017, at 17.40, Jens Axboe <axboe@xxxxxxxxx> wrote:
>>>> 
>>>> On 05/08/2017 09:38 AM, Javier GonzÃlez wrote:
>>>>>> On 8 May 2017, at 17.25, Jens Axboe <axboe@xxxxxxxxx> wrote:
>>>>>> 
>>>>>> On 05/08/2017 09:22 AM, Javier GonzÃlez wrote:
>>>>>>> Javier
>>>>>>> 
>>>>>>>> On 8 May 2017, at 17.14, Jens Axboe <axboe@xxxxxxxxx> wrote:
>>>>>>>> 
>>>>>>>> On 05/08/2017 09:08 AM, Jens Axboe wrote:
>>>>>>>>> On 05/08/2017 09:02 AM, Javier GonzÃlez wrote:
>>>>>>>>>>> On 8 May 2017, at 16.52, Jens Axboe <axboe@xxxxxx> wrote:
>>>>>>>>>>> 
>>>>>>>>>>> On 05/08/2017 08:46 AM, Javier GonzÃlez wrote:
>>>>>>>>>>>>> On 8 May 2017, at 16.23, Jens Axboe <axboe@xxxxxx> wrote:
>>>>>>>>>>>>> 
>>>>>>>>>>>>> On 05/08/2017 08:20 AM, Javier GonzÃlez wrote:
>>>>>>>>>>>>>>> On 8 May 2017, at 16.13, Jens Axboe <axboe@xxxxxx> wrote:
>>>>>>>>>>>>>>> 
>>>>>>>>>>>>>>> On 05/08/2017 07:44 AM, Javier GonzÃlez wrote:
>>>>>>>>>>>>>>>>> On 8 May 2017, at 14.27, Ming Lei <ming.lei@xxxxxxxxxx> wrote:
>>>>>>>>>>>>>>>>> 
>>>>>>>>>>>>>>>>> On Mon, May 08, 2017 at 01:54:58PM +0200, Javier GonzÃlez 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.

I cannot reproduce the latency on a normal nvme drive when mixing I/O
from a fio job and ioctls.

The path is different from the one in pblk, since normal block I/O
uses the generic_make_request(), but still, they both need to
blk_queue_enter(), allocate a request, etc. They only "major" difference
I see is that normal block I/O requests are given by get_request()
(which as far as I understand takes pre-allocated requests from the
queue request list), while pblk allocates each request via
nvme_alloc_request().

What puzzles me most is that having different pblk instances, issuing
I/O in parallel does not trigger the long tail. Otherwise, I would think
that we are just unlucky and get scheduled out. Still, 20ms...

BTW, in order to discard NUMA, I tried on a single socket machine, and
same, same.

Javier.

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