> On 9 May 2017, at 13.21, Javier González <jg@xxxxxxxxxxx> wrote: > >> On 9 May 2017, at 12.58, Ming Lei <ming.lei@xxxxxxxxxx> wrote: >> >> On Tue, May 09, 2017 at 12:34:42PM +0200, Javier González wrote: >>>> On 8 May 2017, at 18.39, Javier González <jg@xxxxxxxxxxx> wrote: >>>> >>>>> 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. >>> >>> I cannot reproduce the latency on a normal nvme drive when mixing I/O >>> from a fio job and ioctls. >>> >>> The path is different from the one in pblk, since normal block I/O >>> uses the generic_make_request(), but still, they both need to >>> blk_queue_enter(), allocate a request, etc. They only "major" difference >>> I see is that normal block I/O requests are given by get_request() >>> (which as far as I understand takes pre-allocated requests from the >>> queue request list), while pblk allocates each request via >>> nvme_alloc_request(). >>> >>> What puzzles me most is that having different pblk instances, issuing >>> I/O in parallel does not trigger the long tail. Otherwise, I would think >>> that we are just unlucky and get scheduled out. Still, 20ms... >>> >>> BTW, in order to discard NUMA, I tried on a single socket machine, and >>> same, same. >> >> I suspect the .q_usage_counter is DEAD, and you can check it via >> percpu_ref_is_dying(), or just check if slow path is reached. >> >> The fast path is that percpu_ref_tryget_live() returns directly, >> and slow path is reached only if queue is freezed or dead. >> >> If that is true, you can add a dump_stack() in blk_freeze_queue_start() >> to see where the unusual freezing/unfreezing is from. > > > Thanks for the hint Ming! You are right. We somehow trigger a re-read > partition: > > [ 324.010184] dump_stack+0x63/0x90 > [ 324.010191] blk_freeze_queue_start+0x1e/0x50 > [ 324.010194] blk_mq_freeze_queue+0x12/0x20 > [ 324.010199] __nvme_revalidate_disk+0xa4/0x350 > [ 324.010203] nvme_revalidate_disk+0x53/0x90 > [ 324.010206] rescan_partitions+0x8d/0x380 > [ 324.010211] ? tlb_flush_mmu_free+0x36/0x60 > [ 324.010218] ? security_capable+0x48/0x60 > [ 324.010221] __blkdev_reread_part+0x65/0x70 > [ 324.010223] blkdev_reread_part+0x23/0x40 > [ 324.010225] blkdev_ioctl+0x387/0x910 > [ 324.010229] ? locks_insert_lock_ctx+0x7e/0xd0 > [ 324.010235] block_ioctl+0x3d/0x50 > [ 324.010239] do_vfs_ioctl+0xa1/0x5d0 > [ 324.010242] ? locks_lock_inode_wait+0x51/0x150 > [ 324.010247] ? kmem_cache_alloc+0xd7/0x1b0 > [ 324.010249] ? locks_alloc_lock+0x1b/0x70 > [ 324.010252] SyS_ioctl+0x79/0x90 > [ 324.010254] ? SyS_flock+0x11c/0x180 > [ 324.010260] entry_SYSCALL_64_fastpath+0x1e/0xad > > I'm checking why this happens now... So apparently we opened a file descriptor with O_RDWR for sending ioctls on liblightnvm. Opening for writing triggered a syscall to re-read and ultimately reached blk_mq_freeze_queue(), which accounted for the latencies we observed. For reference, nvme-cli opens the fd read-only, reason why we could not reproduce the issue with nvme read. Thanks Ming and Jens for looking into this and giving good advice. Much appreciated! Javier
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