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

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Date:   Wed, 8 Feb 2023 09:53:41 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Jan Kara <jack@...e.cz>
Cc:     Hou Tao <houtao@...weicloud.com>, linux-block@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>, cgroups@...r.kernel.org,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, houtao1@...wei.com
Subject: Re: [PATCH] blk-ioprio: Introduce promote-to-rt policy

On 2/8/23 05:43, Jan Kara wrote:
> On Fri 03-02-23 11:45:32, Bart Van Assche wrote:
>> On 2/2/23 17:48, Hou Tao wrote:
>>> I don't get it on how to remove IOPRIO_POL_PROMOTION when calculating the final
>>> ioprio for bio. IOPRIO_POL_PROMOTION is not used for IOPRIO_CLASS values but
>>> used to determinate on how to calculate the final ioprio for bio: choosing the
>>> maximum or minimum between blkcg ioprio and original bio bi_ioprio.
>>
>> Do the block layer code changes shown below implement the functionality
>> that you need?
> 
> Just one question guys: So with my a78418e6a04c ("block: Always initialize
> bio IO priority on submit") none-to-rt policy became effectively a noop as
> Hou properly noticed. Are we aware of any users that were broken by this?
> Shouldn't we rather fix the code so that none-to-rt starts to operate
> correctly again? Or maybe change the none-to-rt meaning to be actually
> promote-to-rt?
> 
> I have to admit I'm wondering a bit what was the intended usecase behind
> the introduction of none-to-rt policy. Can someone elaborate? promote-to-rt
> makes some sense to me - we have a priviledged cgroup we want to provide
> low latency access to IO but none-to-rt just does not make much sense to
> me...

Hi Jan,

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?

Thanks,

Bart.