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. -- Jens Axboe
