> 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. Javier
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