gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

[Andi Kleen]

On Sun, Jan 11, 2009 at 11:25:32AM -0800, Linus Torvalds wrote:
> 
> 
> On Sun, 11 Jan 2009, Andi Kleen wrote:
> > 
> > The proposal was to use -fno-inline-functions-called-once (but 
> > the resulting numbers were not promising)
> 
> Well, the _optimal_ situation would be to not need it, because gcc does a 
> good job without it. That implies trying to find a better balance between 
> "worth it" and "causes problems". 
> 
> Rigth now, it does sound like gcc simply doesn't try to balance AT ALL, or 
> balances only when we add some very version-specific random options (ie 
> the stack-usage one). 

The gcc 4.3 inliner takes stack growth into account by default (without
any special options). I experimented a bit with it when that
was introduced and found the default thresholds are too large for the kernel 
and don't change the checkstack.pl picture much.

I asked back then and was told --param large-stack-frame
is expected to be a reasonable stable --param (as much as these can
be) and I did a patch to lower it, but I couldn't get myself
to actually submit it [if you really want it I can send it]. 

But of course that only helps for gcc 4.3+, older gccs would need
a different workaround.

On the other hand (my personal opinion, not shared by everyone) is 
that the ioctl switch stack issue is mostly only a problem with 4K 
stacks and in the rare cases when I still run 32bit kernels
I never set that option because I consider it russian roulette
(because there undoutedly dangerous dynamic stack growth cases that 
checkstack.pl doesn't flag) 

> And even those options don't actually make much 
> sense - yes, they "balance" things, but they don't do it in a sensible 
> manner.
> 
> For example: stack usage is undeniably a problem (we've hit it over and 
> over again), but it's not about "stack must not be larger than X bytes". 
> 
> If the call is done unconditionally, then inlining _one_ function will 
> grow the static stack usage of the function we inline into, but it will 
> _not_ grow the dynamic stack usage one whit - so deciding to not inline 
> because of stack usage is pointless.

Don't think the current inliner takes that into account from
a quick look at the sources, although it probably could.  
Maybe Honza can comment.

But even if it did it could only do that for a single file,
but if the function is in a different file gcc doesn't
have the information (unless you run with David's --combine hack).
This means the kernel developers have to do it anyways.

On the other hand I'm not sure it's that big a problem. Just someone
should run make checkstack occasionally and add noinlines to large
offenders.

-Andi

[keep quote for Honza's benefit]

> 
> See? So "stop inlining when you hit a stack limit" IS THE WRONG THING TO 
> DO TOO! Because it just means that the compiler continues to do bad 
> inlining decisions until it hits some magical limit - but since the 
> problem isn't the static stack size of any _single_ function, but the 
> combined stack size of a dynamic chain of them, that's totally idiotic. 
> You still grew the dynamic stack, and you have no way of knowing by how 
> much - the limit on the static one simply has zero bearing what-so-ever on 
> the dynamic one.
> 
> So no, "limit static stack usage" is not a good option, because it stops 
> inlining when it doesn't matter (single unconditional call), and doesn't 
> stop inlining when it might (lots of sequential calls, in a deep chain).
> 
> The other alternative is to let gcc do what it does, but 
> 
>  (a) remove lots of unnecessary 'inline's. And we should likely do this 
>      regardless of any "-fno-inline-functions-called-once" issues.
> 
>  (b) add lots of 'noinline's to avoid all the cases where gcc screws up so 
>      badly that it's either a debugging disaster or an actual correctness 
>      issue.
> 
> The problem with (b) is that it's a lot of hard thinking, and debugging 
> disasters always happen in code that you didn't realize would be a problem 
> (because if you had, it simply wouldn't be the debugging issue it is).
> 
> 			Linus
> 

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