linux-kernel - Re: [PATCH v2] blk-ioprio: Introduce promote-to-rt policy

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Date:   Tue, 21 Feb 2023 18:04:03 +0000
From:   Chaitanya Kulkarni <chaitanyak@...dia.com>
To:     Hou Tao <houtao@...weicloud.com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
CC:     Bart Van Assche <bvanassche@....org>, Jan Kara <jack@...e.cz>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        Jens Axboe <axboe@...nel.dk>,
        "cgroups@...r.kernel.org" <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-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "houtao1@...wei.com" <houtao1@...wei.com>
Subject: Re: [PATCH v2] blk-ioprio: Introduce promote-to-rt policy

On 2/20/2023 5:54 AM, Hou Tao wrote:
> From: Hou Tao <houtao1@...wei.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@...wei.com>


Thanks for updating documentation and adding meaningful comment.

Looks good.

Reviewed-by: Chaitanya Kulkarni <kch@...dia.com>

-ck