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. Javier
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