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. -- Jens Axboe
