Re: [GIT PULL] Additional x86 fixes for 2.6.31-rc5

H. Peter Anvin
2009-08-01T19:39:45Z

On 08/01/2009 12:28 PM, Linus Torvalds wrote:
>
> Hmm.
>
> I just noticed another issue on x86 code generation, since I was looking
> at assembly language generation due to the do_sigaltstack() kernel stack
> info leak thing.
>
> Our "get_current()" seriously sucks now that it's a per-cpu variable.
>
> Look at the code generated for something like
>
> current->sas_ss_sp = (unsigned long) ss_sp;
> current->sas_ss_size = ss_size;
>
> and notice how the code really really sucks:
>
> movq %gs:per_cpu__current_task,%rcx
> movq %rdx, 1152(%rcx)
> movq %gs:per_cpu__current_task,%rdx
> movq %rax, 1160(%rdx)
>
> because it reloads that silly per-cpu variable every time, because the
> assembler has a constraint of
>
> "m" (per_cpu__current_task)
>
> and so gcc is worried that the stores will invalidate the result of the
> load from the per-cpu variable.
>
> I don't know how to fix that _well_, but here's a not-so-very-pretty patch
> that seems to shave off 4.5kB from my kernel, and gives gcc much better
> scheduling for 'current' and 'thread_info' because now it can load them
> early - and cache them - even in the presense of stores.
>

This is clearly better... now the semi-obvious question becomes if there
is any way we can get compiler support to do better and migrate to that
as the compiler allows. In particular, if I remember right the problem
with using __thread for percpu was exactly that the current cpuness can
change almost anywhere, unless preemption is disabled.

I'm wondering if we could use __thread or something like it for the
stable perthreads, perhaps with additional compiler hints.

-hpa

--
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.