Sender: Tejun Heo <htejun@gmail.com>
Date:   Tue, 21 Feb 2023 06:37:36 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Hou Tao <houtao@huaweicloud.com>
Cc:     linux-block@vger.kernel.org, Bart Van Assche <bvanassche@acm.org>,
        Jan Kara <jack@suse.cz>, Bagas Sanjaya <bagasdotme@gmail.com>,
        Jens Axboe <axboe@kernel.dk>, cgroups@vger.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 v2] blk-ioprio: Introduce promote-to-rt policy
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On Mon, Feb 20, 2023 at 09:54:28PM +0800, Hou Tao wrote:
> From: Hou Tao <houtao1@huawei.com>
> 
> Since commit a78418e6a04c ("block: Always initialize bio IO priority on
> submit"), bio->bi_ioprio will never be IOPRIO_CLASS_NONE when calling
> blkcg_set_ioprio(), so there will be no way to promote the io-priority
> of one cgroup to IOPRIO_CLASS_RT, because bi_ioprio will always be
> greater than or equals to IOPRIO_CLASS_RT.
> 
> It seems possible to call blkcg_set_ioprio() first then try to
> initialize bi_ioprio later in bio_set_ioprio(), but this doesn't work
> for bio in which bi_ioprio is already initialized (e.g., direct-io), so
> introduce a new ioprio policy to promote the iopriority of bio to
> IOPRIO_CLASS_RT if the ioprio is not already RT.
> 
> So introduce a new promote-to-rt policy to achieve this. For none-to-rt
> policy, although it doesn't work now, but considering that its purpose
> was also to override the io-priority to RT and allow for a smoother
> transition, just keep it and treat it as an alias of the promote-to-rt
> policy.
> 
> Signed-off-by: Hou Tao <houtao1@huawei.com>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun