linux-kernel - Re: Large latency on blk_queue

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Date:   Mon, 8 May 2017 15:44:14 +0200
From:   Javier González <jg@...htnvm.io>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...com>, 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 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?

Thanks,
Javier

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