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    SubjectRe: [PATCH 3/5] x86: Reset the debug_stack update counter
    On 05/30/2012 06:28 PM, Steven Rostedt wrote:
    >>
    > diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c index
    > 9087527..c1fffc5 100644 --- a/arch/x86/kernel/nmi.c +++
    > b/arch/x86/kernel/nmi.c @@ -450,8 +450,10 @@ static inline void
    > nmi_nesting_preprocess(struct pt_regs *regs)
    >
    > static inline void nmi_nesting_postprocess(void) { - if
    > (unlikely(__get_cpu_var(update_debug_stack))) + if
    > (unlikely(__get_cpu_var(update_debug_stack))) {
    > debug_stack_reset(); + __get_cpu_var(update_debug_stack) = 0; + }
    > } #endif
    >

    Please don't use __get_cpu_var(); it causes a pointer to be manifest,
    which is free or almost free on most architectures but quite expensive
    on x86.

    Instead use this_cpu_read()/this_cpu_write().

    Consider:

    #include <linux/types.h>
    #include <linux/percpu.h>

    static DEFINE_PER_CPU(int, percpu_test);

    int read_foo(void)
    {
    return __get_cpu_var(percpu_test);
    }

    void write_foo(int x)
    {
    __get_cpu_var(percpu_test) = x;
    }

    int read_bar(void)
    {
    return this_cpu_read(percpu_test);
    }

    void write_bar(int x)
    {
    this_cpu_write(percpu_test, x);
    }

    ... and the corresponding assembly code (with gcc boilerplate removed):

    read_foo:
    movq $percpu_test, %rax #, tcp_ptr__
    add %gs:this_cpu_off, %rax # this_cpu_off, tcp_ptr__
    movl (%rax), %eax # *D.8429_3, *D.8429_3
    ret

    write_foo:
    movq $percpu_test, %rax #, tcp_ptr__
    add %gs:this_cpu_off, %rax # this_cpu_off, tcp_ptr__
    movl %edi, (%rax) # x, *D.8435_3
    ret

    read_bar:
    movl %gs:percpu_test,%eax # percpu_test, pfo_ret__

    write_bar:
    movl %edi,%gs:percpu_test # x, percpu_test
    ret

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



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