linux-kernel - Re: Large latency on blk_queue

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20170508122738.GC5696@ming.t460p>
Date:   Mon, 8 May 2017 20:27:39 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Javier González <jg@...htnvm.io>
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 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,
Ming