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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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