On Tue, May 09, 2017 at 12:34:42PM +0200, Javier González wrote: > > On 8 May 2017, at 18.39, Javier González <jg@xxxxxxxxxxx> wrote: > > > >> On 8 May 2017, at 18.06, Jens Axboe <axboe@xxxxxxxxx> wrote: > >> > >> On 05/08/2017 09:49 AM, Javier González wrote: > >>>> On 8 May 2017, at 17.40, Jens Axboe <axboe@xxxxxxxxx> wrote: > >>>> > >>>> On 05/08/2017 09:38 AM, Javier González wrote: > >>>>>> On 8 May 2017, at 17.25, Jens Axboe <axboe@xxxxxxxxx> wrote: > >>>>>> > >>>>>> On 05/08/2017 09:22 AM, Javier González 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 González wrote: > >>>>>>>>>>> On 8 May 2017, at 16.52, Jens Axboe <axboe@xxxxxx> wrote: > >>>>>>>>>>> > >>>>>>>>>>> On 05/08/2017 08:46 AM, Javier González wrote: > >>>>>>>>>>>>> On 8 May 2017, at 16.23, Jens Axboe <axboe@xxxxxx> wrote: > >>>>>>>>>>>>> > >>>>>>>>>>>>> On 05/08/2017 08:20 AM, Javier González wrote: > >>>>>>>>>>>>>>> On 8 May 2017, at 16.13, Jens Axboe <axboe@xxxxxx> wrote: > >>>>>>>>>>>>>>> > >>>>>>>>>>>>>>> On 05/08/2017 07:44 AM, Javier González 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 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, Ming
