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



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