Re: [Xen-devel] big local array in routine in hypervisor

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 27/01/2009 08:19, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:

>> Is there one stack per physical processor?  Or is there some
>> other reason (other than historical) for the stack to be so
>> limited in size?
> 
> So you want us to wastefully pre-reserve some space for you, but call it the
> 'stack' to assuage your guilt? ;-) It's common practice not to have very large
> stacks in kernels, since pre-reservation is wateful, dynamic growth is not
> necessarily feasible to implement, and kernel code doesn't tend to need lots
> of local storage or recursion. In Linux you'd be limited to 4kB, and there's a
> lot more code there living under that stricter regime.

I noticed that the p2m populate-on-demand code also allocates a lot (10kB)
of stack (in fact this is a bug since the stack is only 8kB!). If these new
stack users aren't easy to implement in other ways, and are definitely not
reentrant nor execute in interrupt context (so we know there's only one such
big allocation at a time) we could perhaps double the primary stack size to
16kB, or even to 32kB.

It's a slippery slope though, determining how much stack is enough and how
big a local array is too big. I generally end up having to check and fix big
stack frames from time to time, and I'm not sure that even doubling the
stack a few times would avoid that job!

 -- Keir



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