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

[Paolo Valente <paolo.valente@linaro.org>]

> Il giorno 17 gen 2017, alle ore 11:49, Paolo Valente <paolo.valente@linaro.org> ha scritto:
> 
> [NEW RESEND ATTEMPT]
> 
>> Il giorno 17 gen 2017, alle ore 03:47, Jens Axboe <axboe@fb.com> ha scritto:
>> 
>> On 12/22/2016 08:28 AM, Paolo Valente wrote:
>>> 
>>>> Il giorno 19 dic 2016, alle ore 22:05, Jens Axboe <axboe@fb.com> ha scritto:
>>>> 
>>>> 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.
>>>> 
>>>> 
>>> 
>>> 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?
>> 
>> You should be able to do it in get/put_request. But you might need some
>> extra tracking, I'd need to double check.
> 
> Exactly, AFAICT something extra is apparently needed.  In particular,
> get is not ok, because dispatch is a different event (but dispatch is
> however an already controlled event), while put could be used,
> provided that it is guaranteed to be executed only after dispatch.  If
> it is not, then I think that an extra flag or something should be
> added to the request.  I don't know whether adding this extra piece
> would be worst than adding an extra hook.
> 
>> 
>> I'm trying to avoid adding
>> hooks that we don't truly need, the old interface had a lot of that. If
>> you find that you need a hook and it isn't there, feel free to add it.
>> activate/deactivate might be a good change.
>> 
> 
> If my comments above do not trigger any proposal of a better solution,
> then I will try by adding only one extra 'deactivate' hook.  Unless
> unbalanced hooks are a bad idea too.
> 

Jens,
according to the function blk_mq_sched_put_request, the
mq.completed_request hook seems to always be invoked (if set) for a
request for which the mq.put_rq_priv is invoked (if set).

If you don't warn me that I'm wrong, I will base on the above
assumption, and complete bfq without any additional hook or flag.

Thanks,
Paolo

> Thanks,
> Paolo
> 
>> -- 
>> Jens Axboe
> 
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