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, Ming
