From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: Large latency on blk_queue_enter To: =?UTF-8?Q?Javier_Gonz=c3=a1lez?= , Ming Lei References: <1656B440-3ECA-4F2B-B95C-418CF0F347E9@lightnvm.io> <20170508122738.GC5696@ming.t460p> <76E35BA3-FEC9-46D6-B36F-554F464FA9ED@lightnvm.io> Cc: Christoph Hellwig , Dan Williams , linux-block@vger.kernel.org, Linux Kernel Mailing List From: Jens Axboe Message-ID: Date: Mon, 8 May 2017 08:13:56 -0600 MIME-Version: 1.0 In-Reply-To: <76E35BA3-FEC9-46D6-B36F-554F464FA9ED@lightnvm.io> Content-Type: text/plain; charset=windows-1252 List-ID: On 05/08/2017 07:44 AM, Javier Gonz�lez wrote: >> On 8 May 2017, at 14.27, Ming Lei wrote: >> >> On Mon, May 08, 2017 at 01:54:58PM +0200, Javier Gonz�lez 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