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

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Date:   Mon, 19 Dec 2016 19:21:10 +0100
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Jens Axboe <axboe@...com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        Linux-Kernal <linux-kernel@...r.kernel.org>,
        Omar Sandoval <osandov@...com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [PATCHSET v4] blk-mq-scheduling framework


> Il giorno 19 dic 2016, alle ore 16:20, Jens Axboe <axboe@...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@...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?

> Outside of that, some of them have been renamed, and some have been
> collapsed (like activate/deactivate), and yet others again work a little
> differently (like merging). See the mq-deadline conversion, and just
> work through them one at the time.
> 

That's how I'm proceeding, thanks.

Thank you,
Paolo

> -- 
> Jens Axboe
> 
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