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? -- Jens Axboe
