Date:	Mon, 09 Jun 2014 19:58:03 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Junxiao Bi <junxiao.bi@...cle.com>, Andreas Mohr <andi@...as.de>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: make nr_requests tunable for loop

On 2014-06-09 19:35, Junxiao Bi wrote:
> On 06/09/2014 11:53 PM, Jens Axboe wrote:
>> On 2014-06-09 01:29, Andreas Mohr wrote:
>>> Hi,
>>>
>>> having had a look at current mainline sources,
>>> frankly I've (well, initially...) got trouble understanding
>>> what this patch is doing.
>>>
>>> It's replacing an aggressive error-type bail-out (-EINVAL) for NULL
>>> request_fn
>>> with an inoccuous-looking "return ret;", yet that ret content currently
>>> *implicitly* is a >= 0 value (resulting from processing by earlier code
>>> which may or may not get incomprehensibly rewritten in future).
>>> I don't understand the reasons for this huge change in return value
>>> handling
>>> (since it's now not assigning a specific return value
>>> for this modified bail-out case).
>>>
>>> OK, well... you could say that since all this function ever was
>>> interested in is the result value of queue_var_store()
>>> (except for error bail-out cases), doing an interim "return ret;"
>>> (which is exactly what the function tail is also doing)
>>> is exactly right.
>>>
>>> But still simple textual appearance of the resulting patch hunks
>>> seems strangely asymmetric
>>> which may easily be a canary for structurally wrong layering of this
>>> function.
>>> Not to mention the now required extra spin_unlock_irq()
>>> in interim return handler...
>>>
>>>
>>> Well, after further analysis I would come to the conclusion
>>> that in general queue_requests_store() does a LOT more than it should -
>>> since blk-sysfs.c's only (expected!) purpose is
>>> to do parameterization of request_queue behaviour as gathered
>>> from sysfs attribute space,
>>> all that function should ever be concerned with is parsing that sysfs
>>> value
>>> and then calling a blk helper for configuration of that very
>>> attribute value
>>> which would *internally* do all the strange internal queue magic
>>> that is currently being updated *open-coded*
>>> at this supposedly *sysfs*-specific place. Ugh.
>>> Main question here: what would one do if one decided to rip out sysfs
>>> and use something entirely different for parameterization?
>>> Yeah indeed - thought so...
>>>
>>>
>>> So yeah, I'd definitely say that that function is lacking some cleanup
>>> which would possibly then even lead (or: would have led ;)
>>> to a much more nicely symmetric textual appearance
>>> of the patch hunk of the small but quite likely useful change
>>> that you currently intend to have here.
>>
>> If you are done ranting, look at the current tree where it has been
>> split out. There was no reason to have it split before, since the
>> sysfs entry point was the only place where we updated nr_requests. If
>> that code has been duplicated, there would have been a justified
>> reason for writing two pages about it.
> Yes, agree, this is the only place updating nr_requests,  we can split
> it as a separated function if it needs updating at some other places in
> future.

Please look at the current tree... It is already split up.

-- 
Jens Axboe

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