linux-kernel - Re: Large latency on blk_queue

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Date:   Mon, 8 May 2017 08:13:56 -0600
From:   Jens Axboe <axboe@...com>
To:     Javier González <jg@...htnvm.io>,
        Ming Lei <ming.lei@...hat.com>
CC:     Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Large latency on blk_queue_enter

On 05/08/2017 07:44 AM, Javier González wrote:
>> On 8 May 2017, at 14.27, Ming Lei <ming.lei@...hat.com> 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