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

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Date:   Mon, 27 Feb 2023 16:02:23 +0100
From:   Jan Kara <jack@...e.cz>
To:     Hou Tao <houtao@...weicloud.com>
Cc:     Jan Kara <jack@...e.cz>, linux-block@...r.kernel.org,
        Bart Van Assche <bvanassche@....org>,
        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 27-02-23 21:56:25, Hou Tao wrote:
> Hi
> 
> On 2/27/2023 9:03 PM, Jan Kara wrote:
> > 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>
> Thanks for the review.
> >
> > 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?
> These numbers are used in the algorithm paragraph below to explain how the final
> ioprio is calculated. For prompt-to-rt policy, the algorithm is different and
> the number is unnecessary.

I see, thanks for explanation.

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