Date:	Thu, 20 Oct 2011 02:55:19 +0100
From:	Måns Rullgård <mans@...sr.com>
To:	Tim Bird <tim.bird@...sony.com>
Cc:	Måns Rullgård <mans@...sr.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	linux kernel <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arm-kernel\@lists.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 0/3] ARM 4Kstacks: introduction

Tim Bird <tim.bird@...sony.com> writes:

> On 10/19/2011 05:13 PM, � wrote:
>> Tim Bird <tim.bird@...sony.com> writes:
>>> Well, the difference in size is not an order of magnitude.  Current
>>> stacks are only twice as big as what this feature optionally allows.
>>> So people really shouldn't go crazy with their stack usage.
>>>
>>> Granted we're already seeing a bit of stack bloat, but IMHO this should
>>> be nipped in the bud.  If the stack grows a lot more, it will likely
>>> need to shift to 16K, which would REALLY be costly on machines
>>> with lots of threads and low memory.
>> 
>> In binary, 8k is an order of magnitude greater than 4k.
> Touche' ;-)
>
>> Having suffered
>> the consequences of 4k stacks on x86, I have to side with those
>> reluctant to include this.
>
> Can you please provide more detail on what consequences
> you suffered?  I think it would help the discussion.

Random crashes which I eventually figured out were caused by
xfs + device-mapper + md blowing the 4k stack in certain situations.
At the time, 4k stacks were enabled default with the option hidden away
in some obscure place.

> If you had some examples where the burden of supporting 4K stacks
> was onerous, that would be useful.  I know that defaulting it to
> 'Y' introduced lots of subtle bugs that desktop users had issues
> with.  The situation is completely different in deeply embedded,
> because our user-space is completely constrained and controlled.
>
> Also, there are a now a plethora of stack depth monitoring, guarding,
> and reporting tools already in mainline, some of which weren't
> there during the painful era.

The tools have certainly got better since then, but I they are still not
able to guard against every possible failure.

>>> I don't want to give up on a small-footprint Linux so easily,
>>> and the discipline for keeping stacks small is probably good
>>> for code hygiene anyway.
>> 
>> I see your point of view, but an optional setting available only on an
>> architecture not used by the majority is probably not going to have much
>> of an impact on code hygiene.
>> 
>> 4k stacks were tried, and the conclusion was that too much of the kernel
>> simply cannot work within so small a space.  Maintaining a list of safe
>> (or unsafe) combinations is probably not feasible either.
>
> That's not what was in the message of the commit that removed them
> on x86.  Instead it was conjecture about high numbers of threads
> being only applicable to x86_64.
>
> Are you basing the conclusion above on conjecture or measurement?  In
> my configuration, the worst code paths are using approximately 2K of
> stack most of the time, and peak out at about 2.5K.

Someone else in this thread mentioned several combinations known to
require more than 4k.  Layered block devices was one of them.

>>>> If we switch to 4K stacks, the problems of stack usage become ours.
>>>> Do we have the capacity to be able to address these issues as they
>>>> crop up?
>>>
>>> Since it's optional, it would become the problem of the people
>>> who actually use the option.  If you think tracking down stack
>>> abusers is going to be a pain, just don't do it.
>> 
>> It will still be one more thing to sanity check in every bug report.
>> The end result will be more crashes going ignored due to lacking
>> information, and I don't think that's a good thing.
>
> I don't think mainline is overwhelmed with bug reports from
> embedded developers. 

Mainline might not be, but various embedded forums are.  Embedded just
ain't what it used to be.

> Most of the time, embedded devs are so far behind in kernel version
> that they can't reasonably (and don't) report many bugs found.

What makes you say that?  Consumer devices available from shops are a
few versions behind (my Android phone runs 2.6.35), but devices under
development use quite recent kernels.  A few years ago, being stuck with
something hopelessly outdated was the norm, but things have thankfully
improved since then.

> (I know that this is not helping my case, but I'm trying to be honest
> here.)
>
> If 4K stacks were being turned on for the general population of Linux
> users, then I would agree with you.  But this will likely only be
> turned on by a few people, in controlled situations.

The trouble is figuring out which configurations are safe.

> If you think tainting the kernel would help with the bug reporting,
> I'd be OK with that.  We could introduce a "crazy embedded config -
> you're on your own" taint flag.

Taint flags are but a means of automatically ignoring bug reports from
anyone running such a configuration.  They do not make the bugs go away.

>>> I can understand if people don't want to take patches which
>>> unconditionally limit their stack usage - especially if there's
>>> a performance impact. But I don't see much rationale for not
>>> limiting their stack usage conditionally (that is, do something
>>> slightly different on #ifdef CONFIG_4KSTACKS).  If people
>>> object to even that, then I'll maintain those patches out of
>>> tree, if they're really needed.
>> 
>> Having such a conditional increases the maintenance burden for that
>> function.  Changes will be made without testing the 4k alternative,
>> which sooner or later will break, possibly in subtle ways.
>
> Which will affect no one but those who opt in.  I just don't
> think the maintenance burden will be that big.  (At least,
> no one need be bothered by this, unless contacted by someone
> who cares.)

I would certainly not want to be responsible for any code with such
conditionals in it.

>>> Once again, our products work even without such patches now,
>>> based on the configs we're using and the user-space we're running.
>>> Note that this is defaulted to 'N', so no one should be negatively
>>> affected unless they opt in.
>>>
>>> So, I understand your point.  I would, however, like to have
>>> this patch applied, and then deal with any problems as they
>>> come up, rather than preemptively kill the feature on speculative
>>> problems.
>> 
>> An option that will probably, let's be realistic, only be used by you
>> doesn't really belong in mainline.  You are essentially asking everybody
>> else to carry the burden of supporting something only you seem to need.
>
> I'm not so sure that I'm the only one interested in this.
> I'll ask around and see if I can find others.  I'll have
> a good chance at ELC Europe next week.

I'll be there as well.

> But in general this attitude is really depressing.  I can
> remember when support for niche use cases was considered good.

It's always a tradeoff of utility versus overall impact.  The more
obscure a feature is, the more confined it will have to be.  A driver
for some obscure piece of hardware can usually be accommodated without
touching any other code.  The 4k stack feature, however, has the
potential to require changes or impose constraints pretty much
everywhere in the entire kernel.

> I understand the tradeoff if the maintenance burden is high,
> but I'm not convinced it is.

The removal of the 4k stack option on x86 suggests otherwise.

> I don't expect anyone to proactively guard against or remedy stack
> depth issues.  But if they'd be open to accepting patches from those
> who care, that'd be nice.

Of course gratuitous stack bloat should be fixed whenever possible.  I
would never suggest otherwise.  Expecting support for 4k stacks through
an alternate (presumably reduced-functionality) path is however pushing
your luck.  Many maintainers will not even have the hardware necessary
to test this configuration, and not being able to test code I maintain
is not a situation I would like to be in (I am not a maintainer of
anything, so perhaps this carries little weight).

> If you're saying that Linux is only for use on general purpose
> platforms, and deeply embedded doesn't matter, I think that's sad.

I am saying the line has to be drawn somewhere, and it seems that 4k
stacks are pushing things a bit too far.  I would probably support (in
principle, if not with code) an effort to make 4k stacks work
everywhere (possibly with a few well-defined exceptions).  Were it not
for the previous failed attempt, I might even believe it could succeed.

-- 
Måns Rullgård
mans@...sr.com
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