Re: Large latency on blk_queue_enter

Javier González
2017-05-09T11:21:47Z

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> On 9 May 2017, at 12.58, Ming Lei <ming.lei@redhat.com> wrote:
>
> On Tue, May 09, 2017 at 12:34:42PM +0200, Javier González wrote:
>>> On 8 May 2017, at 18.39, Javier González <jg@lightnvm.io> wrote:
>>>
>>>> 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.
>>
>> 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...