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

[Paolo Valente]

> Il giorno 19 dic 2016, alle ore 22:05, Jens Axboe <axboe@xxxxxx> ha scritto:
> 
> On 12/19/2016 11:21 AM, Paolo Valente wrote:
>> 
>>> Il giorno 19 dic 2016, alle ore 16:20, Jens Axboe <axboe@xxxxxx> ha scritto:
>>> 
>>> On 12/19/2016 04:32 AM, Paolo Valente wrote:
>>>> 
>>>>> Il giorno 17 dic 2016, alle ore 01:12, Jens Axboe <axboe@xxxxxx> 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.
> 
> 

Problem: whereas it seems easy and safe to do somewhere else the
simple increment that was done in activate_request, I wonder if it may
happen that a request is deactivate before being completed.  In it may
happen, then, without a deactivate_request hook, the increments would
remain unbalanced.  Or are request completions always guaranteed till
no hw/sw components breaks?

Thanks,
Paolo 

> -- 
> Jens Axboe
> 
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