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    SubjectRe: [PATCH] APICID: Avoid false sharing on the read mostly x86_cpu_to_apicid
    On Tue, 25 Oct 2011, Eial Czerwacki wrote:

    > Avoid false sharing on the read mostly x86_cpu_to_apicid by moving it to
    > __read_mostly section.

    x86_cpu_to_apicid used to be in .init.data and now it's never freed after
    boot?

    > The per-cpu area is write and read and this symbol shows up high on ipi
    > intensive/x86_cpu_to_apicid load.
    >
    > Signed-off-by: Eial Czerwacki<eial@scalemp.com>
    > Signed-off-by: Shai Fultheim<shai@scalemp.com>
    > Author: Ravikiran Thirumalai<kiran@scalex86.org>

    If the author isn't you, then the patch should start with a single line
    that says "From: name <email>" of the author followed by a blank line.
    It's also strange that the author doesn't have a sign-off line.

    Your email client also has corrupted the inline patch, please see
    Documentation/email-clients.txt.

    And, finally, please send this to the x86 maintainers once fixed. See
    ./scripts/get_maintainers.pl <your patch>.

    > Index: b/arch/x86/include/asm/smp.h
    > ===================================================================
    > --- a/arch/x86/include/asm/smp.h 2010-06-01 09:56:03.000000000 -0700
    > +++ b/arch/x86/include/asm/smp.h 2010-06-02 15:59:21.000000000 -0700
    > @@ -36,7 +36,8 @@ static inline struct cpumask *cpu_core_m
    > return per_cpu(cpu_core_map, cpu);
    > }
    >
    > -DECLARE_EARLY_PER_CPU(u16, x86_cpu_to_apicid);
    > +extern u16 x86_cpu_to_apicid[NR_CPUS];
    > +
    > DECLARE_EARLY_PER_CPU(u16, x86_bios_cpu_apicid);
    >
    > /* Static state in head.S used to set up a CPU */
    > @@ -142,7 +143,7 @@ void native_send_call_func_ipi(const str
    > void native_send_call_func_single_ipi(int cpu);
    >
    > void smp_store_cpu_info(int id);
    > -#define cpu_physical_id(cpu) per_cpu(x86_cpu_to_apicid, cpu)
    > +#define cpu_physical_id(cpu) x86_cpu_to_apicid[cpu]
    >
    > /* We don't mark CPUs online until __cpu_up(), so we need another measure */
    > static inline int num_booting_cpus(void)
    > Index: b/arch/x86/kernel/acpi/boot.c
    > ===================================================================
    > --- a/arch/x86/kernel/acpi/boot.c 2010-06-01 09:56:03.000000000 -0700
    > +++ b/arch/x86/kernel/acpi/boot.c 2010-06-02 15:59:21.000000000 -0700
    > @@ -568,7 +568,7 @@ EXPORT_SYMBOL(acpi_map_lsapic);
    >
    > int acpi_unmap_lsapic(int cpu)
    > {
    > - per_cpu(x86_cpu_to_apicid, cpu) = -1;
    > + x86_cpu_to_apicid[cpu] = -1;
    > set_cpu_present(cpu, false);
    > num_processors--;
    >

    Shouldn't this be BAD_APICID? Not sure where we check for -1.


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