Date:   Wed, 8 Feb 2023 14:43:45 +0100
From:   Jan Kara <jack@suse.cz>
To:     Bart Van Assche <bvanassche@acm.org>
Cc:     Hou Tao <houtao@huaweicloud.com>, linux-block@vger.kernel.org,
        Jan Kara <jack@suse.cz>, Jens Axboe <axboe@kernel.dk>,
        cgroups@vger.kernel.org, Tejun Heo <tj@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] blk-ioprio: Introduce promote-to-rt policy
Message-ID: <20230208134345.77bdep3kzp52haxu@quack3>
References: <20230201045227.2203123-1-houtao@huaweicloud.com>
 <8c068af3-7199-11cf-5c69-a523c7c22d9a@acm.org>
 <4f7dcb3e-2d5a-cae3-0e1c-a82bcc3d2217@huaweicloud.com>
 <b6b3c498-e90b-7d1f-6ad5-a31334e433ae@acm.org>
 <beb7782e-72a4-c350-3750-23a767c88753@huaweicloud.com>
 <aedc240d-7c9e-248a-52d2-c9775f3e8ca1@acm.org>
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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@suse.com>
SUSE Labs, CR