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

[akpm]

From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Venkatesh Pallipadi <venkatesh.pallipadi@xxxxxxxxx>
Cc: Len Brown <len.brown@xxxxxxxxx>
Cc: Zhao Yakui <yakui.zhao@xxxxxxxxx>
Cc: Dave Jones <davej@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Mike Galbraith <efault@xxxxxx>
Cc: "Zhang, Yanmin" <yanmin_zhang@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 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)
_
--
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