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? 2) What did you use to time the stall being blk_queue_enter()? -- Jens Axboe
