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

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Message-Id: <200904141821.37413.rusty@rustcorp.com.au>
Date:	Tue, 14 Apr 2009 18:21:36 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Dave Jones <davej@...hat.com>, lenb@...nel.org,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
	efault@....de, len.brown@...el.com, mingo@...e.hu,
	tglx@...utronix.de, venkatesh.pallipadi@...el.com,
	yakui.zhao@...el.com, yanmin_zhang@...ux.intel.com
Subject: Re: [patch for 2.6.30 2/2] arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c: avoid cross-CPU interrupts

On Sun, 12 Apr 2009 10:16:44 am Andrew Morton wrote:
> I suspect that changing cpumask_any() to preferentially return this-cpu
> will always give us the behaviour that we prefer, but I haven't looked
> into it.

How's this?

Subject: cpumask: cpumask_closest()

Impact: new function

Andrew points out that acpi-cpufreq uses cpumask_any, when it really
would prefer to use the same CPU if possible (to avoid an IPI).  In 
general, this seems a good idea to offer.

Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
CC: Andrew Morton <akpm@...ux-foundation.org>

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -931,6 +931,8 @@ static inline void cpumask_copy(struct c
  */
 #define cpumask_of(cpu) (get_cpu_mask(cpu))
 
+unsigned int cpumask_closest(const struct cpumask *mask);
+
 /**
  * cpumask_scnprintf - print a cpumask into a string as comma-separated hex
  * @buf: the buffer to sprintf into
diff --git a/lib/cpumask.c b/lib/cpumask.c
--- a/lib/cpumask.c
+++ b/lib/cpumask.c
@@ -170,3 +170,26 @@ void __init free_bootmem_cpumask_var(cpu
 	free_bootmem((unsigned long)mask, cpumask_size());
 }
 #endif
+
+/**
+ * cpumask_closest - return the closest cpu in mask.
+ * @mask: the cpus to choose from.
+ *
+ * Returns >= nr_cpu_ids if no bits are set in @mask.
+ */
+unsigned int cpumask_closest(const struct cpumask *mask)
+{
+	unsigned int cpu = raw_smp_processor_id();
+
+	/* Try for same CPU. */
+	if (cpumask_test_cpu(cpu, mask))
+		return cpu;
+
+	/* Try for same node. */
+	cpu = cpumask_any_and(cpumask_of_node(cpu), mask);
+	if (cpu <= nr_cpu_ids)
+		return cpu;
+
+	/* Anything will do. */
+	return cpumask_any(mask);
+}
--
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