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    SubjectRe: [patch for 2.6.30 2/2] arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c: avoid cross-CPU interrupts


    > From: Andrew Morton <akpm@linux-foundation.org>
    >
    > In drv_read(), check to see whether we can run the rdmsr() on the current
    > CPU. If so, do that. So smp_call_function_single() can avoid the IPI.
    >
    > Arguably, cpumask_any() should do this.
    >
    > Cc: Rusty Russell <rusty@rustcorp.com.au>
    > Cc: Ingo Molnar <mingo@elte.hu>
    > Cc: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
    > Cc: Len Brown <len.brown@intel.com>
    > Cc: Zhao Yakui <yakui.zhao@intel.com>
    > Cc: Dave Jones <davej@redhat.com>
    > Cc: Thomas Gleixner <tglx@linutronix.de>
    > Tested-by: Mike Galbraith <efault@gmx.de>
    > Cc: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
    > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    >
    > ---
    >
    > arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c | 17 +++++++++++++++--
    > 1 file changed, 15 insertions(+), 2 deletions(-)
    >
    > diff -puN arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c~arch-x86-kernel-cpu-cpufreq-acpi-cpufreqc-avoid-cross-cpu-interrupts arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c
    > --- a/arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c~arch-x86-kernel-cpu-cpufreq-acpi-cpufreqc-avoid-cross-cpu-interrupts
    > +++ a/arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c
    > @@ -197,9 +197,22 @@ static void do_drv_write(void *_cmd)
    >
    > static void drv_read(struct drv_cmd *cmd)
    > {
    > - cmd->val = 0;
    > + int target_cpu; /* The CPU on which to perform thr rdmsr() */
    > + int this_cpu;
    > +
    > + /*
    > + * If the current CPU is in cmd->mask then run the rdmsr() on this
    > + * CPU to avoid the cross-cpu interrupt.
    > + */
    > + this_cpu = get_cpu();
    > + if (cpu_isset(this_cpu, *(cmd->mask)))
    > + target_cpu = this_cpu;
    > + else
    > + target_cpu = cpumask_any(cmd->mask);
    >
    > - smp_call_function_single(cpumask_any(cmd->mask), do_drv_read, cmd, 1);
    > + cmd->val = 0;
    > + smp_call_function_single(target_cpu, do_drv_read, cmd, 1);
    > + put_cpu();
    > }
    >
    > static void drv_write(struct drv_cmd *cmd)
    > _

    Rather than this patch I would expect we would want to either:

    A. as we went to the trouble to detect the local case
    in drv_read, why call smp_call_function at all for that case?

    or

    B. optimize smp_call_function_single to beneift all users
    instead of just this customer.

    thanks,
    -Len Brown, Intel Open Source Technology Center



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