> 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... Javier
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