Date: Sat, 11 Oct 2008 19:56:17 +0200
From: Jens Axboe <jens.axboe@oracle.com>
To: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Cc: petkovbb@gmail.com, Robert Hancock <hancockr@shaw.ca>,
       linux-ide@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/7] ide: locking improvements
Message-ID: <20081011175617.GX19428@kernel.dk>
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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.

> [ 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.

> > > > > > > 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
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).

-- 
Jens Axboe

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