> 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. Thanks! Javier
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