> On 8 May 2017, at 18.39, Javier González <j...@lightnvm.io> wrote: > >> On 8 May 2017, at 18.06, Jens Axboe <ax...@kernel.dk> wrote: >> >> On 05/08/2017 09:49 AM, Javier González wrote: >>>> On 8 May 2017, at 17.40, Jens Axboe <ax...@kernel.dk> wrote: >>>> >>>> On 05/08/2017 09:38 AM, Javier González wrote: >>>>>> On 8 May 2017, at 17.25, Jens Axboe <ax...@kernel.dk> wrote: >>>>>> >>>>>> On 05/08/2017 09:22 AM, Javier González wrote: >>>>>>> Javier >>>>>>> >>>>>>>> On 8 May 2017, at 17.14, Jens Axboe <ax...@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 <ax...@fb.com> wrote: >>>>>>>>>>> >>>>>>>>>>> On 05/08/2017 08:46 AM, Javier González wrote: >>>>>>>>>>>>> On 8 May 2017, at 16.23, Jens Axboe <ax...@fb.com> wrote: >>>>>>>>>>>>> >>>>>>>>>>>>> On 05/08/2017 08:20 AM, Javier González wrote: >>>>>>>>>>>>>>> On 8 May 2017, at 16.13, Jens Axboe <ax...@fb.com> wrote: >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> On 05/08/2017 07:44 AM, Javier González wrote: >>>>>>>>>>>>>>>>> On 8 May 2017, at 14.27, Ming Lei <ming....@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. Javier.
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