> 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? Thanks, Javier
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