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

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Message-ID: <20230208134345.77bdep3kzp52haxu@quack3>
Date:   Wed, 8 Feb 2023 14:43:45 +0100
From:   Jan Kara <jack@...e.cz>
To:     Bart Van Assche <bvanassche@....org>
Cc:     Hou Tao <houtao@...weicloud.com>, linux-block@...r.kernel.org,
        Jan Kara <jack@...e.cz>, 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 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...

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