Re: Large latency on blk_queue_enter

[Javier GonzÃlez]

> On 9 May 2017, at 13.21, Javier GonzÃlez <jg@xxxxxxxxxxx> wrote:
> 
>> On 9 May 2017, at 12.58, Ming Lei <ming.lei@xxxxxxxxxx> wrote:
>> 
>> On Tue, May 09, 2017 at 12:34:42PM +0200, Javier GonzÃlez wrote:
>>>> 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.
>> 
>> I suspect the .q_usage_counter is DEAD, and you can check it via
>> percpu_ref_is_dying(), or just check if slow path is reached.
>> 
>> The fast path is that percpu_ref_tryget_live() returns directly,
>> and slow path is reached only if queue is freezed or dead.
>> 
>> If that is true, you can add a dump_stack() in blk_freeze_queue_start()
>> to see where the unusual freezing/unfreezing is from.
> 
> 
> Thanks for the hint Ming! You are right. We somehow trigger a re-read
> partition:
> 
> [  324.010184]  dump_stack+0x63/0x90
> [  324.010191]  blk_freeze_queue_start+0x1e/0x50
> [  324.010194]  blk_mq_freeze_queue+0x12/0x20
> [  324.010199]  __nvme_revalidate_disk+0xa4/0x350
> [  324.010203]  nvme_revalidate_disk+0x53/0x90
> [  324.010206]  rescan_partitions+0x8d/0x380
> [  324.010211]  ? tlb_flush_mmu_free+0x36/0x60
> [  324.010218]  ? security_capable+0x48/0x60
> [  324.010221]  __blkdev_reread_part+0x65/0x70
> [  324.010223]  blkdev_reread_part+0x23/0x40
> [  324.010225]  blkdev_ioctl+0x387/0x910
> [  324.010229]  ? locks_insert_lock_ctx+0x7e/0xd0
> [  324.010235]  block_ioctl+0x3d/0x50
> [  324.010239]  do_vfs_ioctl+0xa1/0x5d0
> [  324.010242]  ? locks_lock_inode_wait+0x51/0x150
> [  324.010247]  ? kmem_cache_alloc+0xd7/0x1b0
> [  324.010249]  ? locks_alloc_lock+0x1b/0x70
> [  324.010252]  SyS_ioctl+0x79/0x90
> [  324.010254]  ? SyS_flock+0x11c/0x180
> [  324.010260]  entry_SYSCALL_64_fastpath+0x1e/0xad
> 
> I'm checking why this happens now...

So apparently we opened a file descriptor with O_RDWR for sending ioctls
on liblightnvm. Opening for writing triggered a syscall to re-read
and ultimately reached blk_mq_freeze_queue(), which accounted for the
latencies we observed. For reference, nvme-cli opens the fd read-only,
reason why we could not reproduce the issue with nvme read.

Thanks Ming and Jens for looking into this and giving good advice. Much
appreciated!

Javier

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