Re: [RFC PATCH 2/2] kernel/sched.c: VLA in middle of struct

From: Ingo Molnar
Date: Tue May 12 2009 - 04:55:49 EST



* Jeff Garzik <jeff@xxxxxxxxxx> wrote:

> Ingo Molnar wrote:
>>>>>> That cpumask[] should probably be cpumask[0], to document the
>>>>>> aliasing to ->span and ->cpus properly.
>>>>> If the comment wasn't sufficient documentation, I don't think
>>>>> that would help :(
>>>> It's a visual helper: it matches up with how we do these 'zero size
>>>> array means dynamic structure continuation' tricks generally.
>>>>
>>>> I first mis-parsed the code for a second when seeing cpumask[].
>>>> cpumask[0] stands out like a sore thumb. And we dont read comments
>>>> anyway ;-)
>>>>
>>>> Jeff, i suspect you found this because you are working on something
>>>> rather interesting? :) If yes, would it help your project if we
>>>> did the cpumask[0] cleanup and pushed it upstream immediately?
>>> I think cpumask[0] would be more clear and consistent with the rest
>>> of the kernel.
>>>
>>> But unfortunately for the twin projects of (a) static analysis and
>>> checking with 'sparse', and (b) compiling under another compiler,
>>> VLA-in-middle-of-struct is a killer in either case.
>>
>> even if at the end of the struct?
>
> Putting the VLA at the end of the struct would be a huge help,
> yes.
>
> For example, struct sched_group and struct sched_domain are OK
> as-is (though "[0]" would be preferred).
>
> It is the definition of struct static_sched_group and struct
> static_sched_domain that creates the problem, because with the
> bitmap following cpumask[] and span[], the VLA is no longer at the
> end of the struct.
>
> VLA-in-the-middle raises the complexity required of the compiler
> quite a bit. As a result, VLA-in-middle is not implemented in
> sparse or clang (LLVM's C front-end and static analyzer).

feel free to send patches for this - i dont have those build modes
to test that it's sufficient. I'd suggest to go the simplest path:
remove all the vla aliasing tricks: just make struct sched_domain
use a plain struct cpumask and eliminate static_sched_domain
altogether.

The memory overhead is marginal as most of our sched domains are
static allocated (and full size) anyway.

Ingo
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