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

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Message-ID: <20230227130305.2idxwmz2kdnacolc@quack3>
Date:   Mon, 27 Feb 2023 14:03:05 +0100
From:   Jan Kara <jack@...e.cz>
To:     Hou Tao <houtao@...weicloud.com>
Cc:     linux-block@...r.kernel.org, Bart Van Assche <bvanassche@....org>,
        Jan Kara <jack@...e.cz>, Bagas Sanjaya <bagasdotme@...il.com>,
        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 v2] blk-ioprio: Introduce promote-to-rt policy

On Mon 20-02-23 21:54:28, 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>

Looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

Just one question regarding doc below:

> ++----------------+---+
> +| no-change      | 0 |
> ++----------------+---+
> +| rt-to-be       | 2 |
> ++----------------+---+
> +| all-to-idle    | 3 |
> ++----------------+---+

Shouldn't there be preempt-to-rt somewhere in this table as well? Or why
this this in the doc at all? I'd consider the numbers to be kernel internal
thing?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR