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

[Jens Axboe]

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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