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SubjectRe: [patch for 2.6.30 2/2] arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c: avoid cross-CPU interrupts
On Tue, 14 Apr 2009 18:21:36 +0930 Rusty Russell <rusty@rustcorp.com.au> wrote:

> 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@rustcorp.com.au>
> CC: Andrew Morton <akpm@linux-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);
> +}

Should it be exported?

It looks all racy against hotplug. What are the caller's
responsibilities here?

<greps a bit>

any_online_cpu() could use cpumask_closest(), against (*mask & cpu_online_map).

I think all cpumask_any() call sites can be migrated to
cpumask_closest() with, at worst, no benefit.



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