Re: [PATCH 4/8] block/mq-deadline: Only use zone locking if necessary

From: Jens Axboe <axboe@kernel.dk>
To: Damien Le Moal <damien.lemoal@opensource.wdc.com>,
	Bart Van Assche <bvanassche@acm.org>
Cc: linux-block@vger.kernel.org, Jaegeuk Kim <jaegeuk@kernel.org>,
	Avri Altman <avri.altman@wdc.com>,
	Damien Le Moal <damien.lemoal@wdc.com>
Subject: Re: [PATCH 4/8] block/mq-deadline: Only use zone locking if necessary
Date: Mon, 9 Jan 2023 20:00:02 -0700	[thread overview]
Message-ID: <ed255a4a-a0da-a962-2da4-13321d0a75c5@kernel.dk> (raw)
In-Reply-To: <873cbbef-e01f-3142-af2d-053dc040d17e@opensource.wdc.com>

On 1/9/23 7:24?PM, Damien Le Moal wrote:
> On 1/10/23 09:48, Jens Axboe wrote:
>> On 1/9/23 5:44?PM, Bart Van Assche wrote:
>>> On 1/9/23 16:41, Jens Axboe wrote:
>>>> Or, probably better, a stacked scheduler where the bottom one can be zone
>>>> away. Then we can get rid of littering the entire stack and IO schedulers
>>>> with silly blk_queue_pipeline_zoned_writes() or blk_is_zoned_write() etc.
>>>
>>> Hi Jens,
>>>
>>> Isn't one of Damien's viewpoints that an I/O scheduler should not do
>>> the reordering of write requests since reordering of write requests
>>> may involve waiting for write requests, write request that will never
>>> be received if all tags have been allocated?
>>
>> It should be work conservering, it should not wait for anything. If
>> there are holes or gaps, then there's nothing the scheduler can do.
>>
>> My point is that the strict ordering was pretty hacky when it went in,
>> and rather than get better, it's proliferating. That's not a good
>> direction.
> 
> Yes, and hard to maintain/avoid breaking something.

Indeed! It's both fragile and ends up adding branches in a bunch of
spots in the generic code, which isn't ideal either from an efficiency
pov.

> Given that only writes need special handling, I am thinking that having a
> dedicated write queue for submission/scheduling/requeue could
> significantly clean things up. Essentially, we would have a different code
> path for zoned device write from submit_bio(). Something like:
> 
> if (queue_is_zoned() && op_is_write())
> 	return blk_zoned_write_submit();
> 
> at the top of submit_bio(). That zone write code can be isolated in
> block/blk-zoned.c and avoid spreading "if (zoned)" all over the place.
> E.g. the flush machinery reorders writes right now... That needs fixing,
> more "if (zoned)" coming...
> 
> That special zone write queue could also do its own dispatch scheduling,
> so no need to hack existing schedulers.

This seems very reasonable, and would just have the one check at queue
time, and then one at requeue time (which is fine, that's not a fast
path in any case).

-- 
Jens Axboe