Message-Id: <200810112046.23557.bzolnier@gmail.com>
Date:	Sat, 11 Oct 2008 20:46:23 +0200
From:	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	petkovbb@...il.com, Robert Hancock <hancockr@...w.ca>,
	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] ide: locking improvements

On Saturday 11 October 2008, Jens Axboe wrote:
> On Sat, Oct 11 2008, Bartlomiej Zolnierkiewicz wrote:
> > On Saturday 11 October 2008, Jens Axboe wrote:
> > > On Sat, Oct 11 2008, Bartlomiej Zolnierkiewicz wrote:
> > > > On Saturday 11 October 2008, Jens Axboe wrote:
> > > > > On Sat, Oct 11 2008, Borislav Petkov wrote:
> > > > > > > >From my perspective the main gain of these patches is the increased
> > > > > > > maintainability and sanity of the code, scalability improvements are
> > > > > > > just an added bonus.
> > > > > > 
> > > > > > and better code/improved scalability is a bad thing because... ?!
> > > > > 
> > > > > It's a bad thing because nobody on earth cares about IDE scalability,
> > > > 
> > > > JFYI: just yesterday I got mail proving otherwise. ;)
> > > 
> > > Well, there are lots of crazy people in the world, a request from
> > > someone doesn't necessarily make it a good idea :-)
> > > 
> > > > > from a performance POV a modern SATA controller is just better on
> > > > > several levels. I don't think anybody cares about IDE scaling on 8-16
> > > > > cores or more, simply because NOBODY is using IDE on such systems.
> > > > > 
> > > > > As such, trying to improve locking is a pointless exercise. And that is
> > > > > a bad thing, because code change invariably brings in code bugs. Then
> > > > > see previous mail on lack of coverage testing, and it can naturally be
> > > > > harmful.
> > > > 
> > > > Your concerns were already addressed in my reply but I worry that having
> > > > a discussion based on technical arguments is not your goal.
> > > 
> > > You make it sound like I have an alterior motive, which I definitely do
> > > not. I just wondered what all the IDE churn was supposed to be good
> > > for...
> > > 
> > > > Just to repeat: these patches are not hardware specific and obviously
> > > > they are not going to be merged today, tomorrow or in a week (they are
> > > > 2.6.29 material after months of time in pata tree / linux-next).
> > > 
> > > It's less about this specific patchset than in general. The specific one
> > > looked fine by itself, it's just the path to to 'IDE lock scaling' that
> > > is a bit crazy to me. Moving IDE to the softirq completion like SCSI
> > > would be a better start, imho. IDE still does most of the processing
> > 
> > We are getting there but this can't be done without fixing some other
> > stuff (like the patch posted today to not process the next request from
> > hard-IRQ context).  Any help would be much appreciated.
> > 
> > > under its ide_lock, which isn't even necessary. Making the locking more
> > 
> > Well, actually it doesn't do most work under ide_lock (never has been)
> > as the main means of protection is hwgroup->busy (which is protected by
> > ide_lock).
> 
> Yes it does, it does all of IO processing under the ide_lock, where you
> only really need the lock for actually putting the last reference to the
> request.

When it comes to IO processing (block layer level not hardware one) you
are of course right.  I think that this patchset addresses most of it but
it would be great if you could double check and fix the places that I
missed.

> > [ OTOH some changes in this patchset were inspired by _your_ comments about
> >   "room for locking improvements" in ide-io.c (IIRC) so kudos to you! ]
> 
> And that is indeed what that comment was about :-)
> There's certainly a way to make that behave a lot more nicely without
> splitting the lock. It's more about latency than lock contention.
> 
> > > finely grained is what I think is pretty crazy.
> > 
> > The more fine grained locking changes that were posted in separate patch
> > were first done 3 years ago by Scalex86 guys and they were extensively
> > tested on Intel hardware (however since other host drivers and things
> > like IDE settings were abusing ide_lock applying this change back than
> > was impossible - all of these stuff is fixed in the current Linus' tree).
> > 
> > Sorry for not explaining this clearly in the changelog.
> 
> Yeah I did get that reference, I am still questioning the point of
> actually doing that.

The work has already been done and it is a wortwhile work.  The risk is
quite low (this is the statement based on rather deep understanding of
IDE subsystem, the complete audit of all code-paths affected and all the
testing experiences from Scalex86/me).

Moreover the patch won't be merged after few months of extra testing.

I feel that you still keep on questioning the point of improving IDE
and insist on putting it into "bug-fixes only" mode.  If this is really
the case I'm completely uninterested in discussing it any further.

> > > > > > > > rather like putting makeup on a corpse to me..
> > > > > > 
> > > > > > so _NOT_ true.
> > > > > 
> > > > > Depends on what you think is the corpse. Since IDE is essentially dead
> > > > > and frozen, it IS a corpse and the phrase is then very appropriate. This
> > > > > is not a personal jab at the IDE guys and does not reflect on the
> > > > > (mostly) good work they do, just a reflection on the state of IDE in
> > > > > general.
> > > > 
> > > > Interesting statement given that i.e. diffstat-wise pata tree has more
> > > > than twice as much stuff queued up for 2.6.28 than "some other" trees
> > > > (and we have history of being a _very_ conservative w.r.t. to needlessly
> > > > moving code around in drivers/ide/).
> > > > 
> > > > Please stop being silly and pushing your view/idea on what other people
> > > > should be doing (not to mention ignoring real facts).
> > > 
> > > Please start by actually _reading_ what I write. Your reply is based on
> > > the code base, my statement pertains to IDE on the hardware side
> > > (devices, controllers, etc). On the scalability front, what new hardware
> > > do you envision shipping with IDE controllers that are actually used for
> > > pushing large amounts of data? People would have to be borderline insane
> > > to make such new hw today.
> > 
> > Please understand that we are not doing new-hardware-driven-development
> > here but rather a big maintainance project.  Like I said in reply to Robert
> > the scalability improvements are an added bonus from my POV -> the main
> > goal is making the code simpler, harder to abuse and easier to maintain.
> 
> I do understand that, as I've said all along I'm more concerned with
> coverage of testing since a lot of the supported hardware (not just low
> level drivers, things like ide-tape) just isn't used a whole lot these
> days. Or the last 5 years, even.
> 
> > > I'm not pushing my views on anyone, but I am free to share what I
> > > actually think. I actually care about old code stability, hence my
> > > concern with the amount of IDE changes.
> > 
> > The actual users' feedback about amount of IDE changes have been mostly
> > positive (some ask for even more changes :) so I don't think that it
> > should be a reason of a great worries.  I hope that all other concerns
> > have been also cleared now.
> 
> Heck that's good, I do hope that I'm mostly over-concerned! I'm not
> trying to create a problem where there isn't one, mainly looking for
> clarity into the situation.
> 
> I noticed one ide-tape tested complaining something broke, and it seems
> like he was the only one out there actually using current kernels and

ide-tape is quite a special case since it is on life support since early
2.6.x days (when I ressurected it from somebody's broken bio changes) and
Borislav/me put quite a lot of work into keeping it alive despite having
real difficulties with finding people willing to test changes (fortunately
things seem to be improving here).

> testing it. I worry mostly about the changes breaking somebodys distro 3
> years down the line, by which point people may have moved on (and the
> old code would have worked).

I'm not quite sure I get it here.

Do you mean that we should be more worried about things like:

http://bugzilla.kernel.org/show_bug.cgi?id=11581

?

Thanks,
Bart
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