Re: [PATCH] block: make nr_requests tunable for loop

[Junxiao Bi <junxiao.bi@oracle.com>]

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.