Re: [PATCH] blk-ioprio: Introduce promote-to-rt policy

From: Bart Van Assche <bvanassche@acm.org>
To: Jan Kara <jack@suse.cz>
Cc: Hou Tao <houtao@huaweicloud.com>,
	linux-block@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
	cgroups@vger.kernel.org, Tejun Heo <tj@kernel.org>,
	Zefan Li <lizefan.x@bytedance.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Jonathan Corbet <corbet@lwn.net>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	houtao1@huawei.com
Subject: Re: [PATCH] blk-ioprio: Introduce promote-to-rt policy
Date: Thu, 9 Feb 2023 11:09:33 -0800	[thread overview]
Message-ID: <55a065e7-7d86-d58f-15ba-c631a427843e@acm.org> (raw)
In-Reply-To: <20230209085603.dzqfcc3pp4hacqlz@quack3>

On 2/9/23 00:56, Jan Kara wrote:
> On Wed 08-02-23 09:53:41, Bart Van Assche wrote:
>> The test results I shared some time ago show that IOPRIO_CLASS_NONE was the
>> default I/O priority two years ago (see also https://lore.kernel.org/linux-block/20210927220328.1410161-5-bvanassche@acm.org/).
>> The none-to-rt policy increases the priority of bio's that have not been
>> assigned an I/O priority to RT. Does this answer your question?
> 
> Not quite. I know that historically we didn't set bio I/O priority in some
> paths (but we did set it in other paths such as some (but not all) direct
> IO implementations). But that was exactly a mess because how none-to-rt
> actually behaved depended on the exact details of the kernel internal IO
> path.  So my question is: Was none-to-rt actually just a misnomer and the
> intended behavior was "always override to RT"? Or what was exactly the
> expectation around when IO priority is not set and should be overridden?
> 
> How should it interact with AIO submissions with IOCB_FLAG_IOPRIO? How
> should it interact with task having its IO priority modified with
> ioprio_set(2)? And what if task has its normal scheduling priority modified
> but that translates into different IO priority (which happens in
> __get_task_ioprio())?
> 
> So I think that none-to-rt is just poorly defined and if we can just get
> rid of it (or redefine to promote-to-rt), that would be good. But maybe I'm
> missing some intended usecase...

Hi Jan,

We have no plans to use the ioprio_set() system call since it only 
affects foreground I/O and not page cache writeback.

While Android supports io_uring, there are no plans to support libaio in 
the Android C library (Bionic).

Regarding __get_task_ioprio(), I haven't found any code in that function 
that derives an I/O priority from the scheduling priority. Did I perhaps 
overlook something?

Until recently "none-to-rt" meant "if no I/O priority has been assigned 
to a task, use IOPRIO_CLASS_RT". Promoting the I/O priority to 
IOPRIO_CLASS_RT works for us. I'm fine with changing the meaning of 
"none-to-rt" into promoting the I/O priority class to RT. Introducing 
"promote-to-rt" as a synonym of "none-to-rt" is also fine with me.

Thanks,

Bart.