Re: [PATCHSET v4] blk-mq-scheduling framework

From: Jens Axboe <axboe@fb.com>
To: Paolo Valente <paolo.valente@linaro.org>
Cc: Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org,
	Linux-Kernal <linux-kernel@vger.kernel.org>,
	Omar Sandoval <osandov@fb.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	Ulf Hansson <ulf.hansson@linaro.org>,
	Mark Brown <broonie@kernel.org>
Subject: Re: [PATCHSET v4] blk-mq-scheduling framework
Date: Mon, 19 Dec 2016 14:05:25 -0700	[thread overview]
Message-ID: <f5303f81-811a-8d0a-b271-d091d8839778@fb.com> (raw)
In-Reply-To: <CF1D593B-8BD0-4B59-9193-9D1D4E8BE3E4@linaro.org>

On 12/19/2016 11:21 AM, Paolo Valente wrote:
> 
>> Il giorno 19 dic 2016, alle ore 16:20, Jens Axboe <axboe@fb.com> ha scritto:
>>
>> On 12/19/2016 04:32 AM, Paolo Valente wrote:
>>>
>>>> Il giorno 17 dic 2016, alle ore 01:12, Jens Axboe <axboe@fb.com> ha scritto:
>>>>
>>>> This is version 4 of this patchset, version 3 was posted here:
>>>>
>>>> https://marc.info/?l=linux-block&m=148178513407631&w=2
>>>>
>>>> From the discussion last time, I looked into the feasibility of having
>>>> two sets of tags for the same request pool, to avoid having to copy
>>>> some of the request fields at dispatch and completion time. To do that,
>>>> we'd have to replace the driver tag map(s) with our own, and augment
>>>> that with tag map(s) on the side representing the device queue depth.
>>>> Queuing IO with the scheduler would allocate from the new map, and
>>>> dispatching would acquire the "real" tag. We would need to change
>>>> drivers to do this, or add an extra indirection table to map a real
>>>> tag to the scheduler tag. We would also need a 1:1 mapping between
>>>> scheduler and hardware tag pools, or additional info to track it.
>>>> Unless someone can convince me otherwise, I think the current approach
>>>> is cleaner.
>>>>
>>>> I wasn't going to post v4 so soon, but I discovered a bug that led
>>>> to drastically decreased merging. Especially on rotating storage,
>>>> this release should be fast, and on par with the merging that we
>>>> get through the legacy schedulers.
>>>>
>>>
>>> I'm to modifying bfq.  You mentioned other missing pieces to come.  Do
>>> you already have an idea of what they are, so that I am somehow
>>> prepared to what won't work even if my changes are right?
>>
>> I'm mostly talking about elevator ops hooks that aren't there in the new
>> framework, but exist in the old one. There should be no hidden
>> surprises, if that's what you are worried about.
>>
>> On the ops side, the only ones I can think of are the activate and
>> deactivate, and those can be done in the dispatch_request hook for
>> activate, and put/requeue for deactivate.
>>
> 
> You mean that there is no conceptual problem in moving the code of the
> activate interface function into the dispatch function, and the code
> of the deactivate into the put_request? (for a requeue it is a little
> less clear to me, so one step at a time)  Or am I missing
> something more complex?

Yes, what I mean is that there isn't a 1:1 mapping between the old ops
and the new ops. So you'll have to consider the cases.

-- 
Jens Axboe