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    SubjectRe: [PATCH] Avoid mask based num_possible_cpus and num_online_cpus
    (1/17/12 9:07 PM), Venkatesh Pallipadi wrote:
    > Kernel's notion of possible cpus (from include/linux/cpumask.h)
    > * cpu_possible_mask- has bit 'cpu' set iff cpu is populatable
    >
    > * The cpu_possible_mask is fixed at boot time, as the set of CPU id's
    > * that it is possible might ever be plugged in at anytime during the
    > * life of that system boot.
    >
    > #define num_possible_cpus() cpumask_weight(cpu_possible_mask)
    >
    > and on x86 cpumask_weight() calls hweight64 and hweight64 (on older kernels
    > and systems with !X86_FEATURE_POPCNT) or a popcnt based alternative.
    >
    > i.e, We needlessly go through this mask based calculation everytime
    > num_possible_cpus() is called.
    >
    > The problem is there with cpu_online_mask() as well, which is fixed value at
    > boot time in !CONFIG_HOTPLUG_CPU case and should not change that often even
    > in HOTPLUG case.
    >
    > Though most of the callers of these two routines are init time (with few
    > exceptions of runtime calls), it is cleaner to use variables
    > and not go through this repeated mask based calculation.
    >
    > Signed-off-by: Venkatesh Pallipadi<venki@google.com>
    > ---
    > include/linux/cpumask.h | 8 ++++++--
    > kernel/cpu.c | 9 +++++++++
    > 2 files changed, 15 insertions(+), 2 deletions(-)
    >
    > diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
    > index 4f7a632..2eb04dd 100644
    > --- a/include/linux/cpumask.h
    > +++ b/include/linux/cpumask.h
    > @@ -80,9 +80,13 @@ extern const struct cpumask *const cpu_online_mask;
    > extern const struct cpumask *const cpu_present_mask;
    > extern const struct cpumask *const cpu_active_mask;
    >
    > +extern int nr_online_cpus;
    > +
    > #if NR_CPUS> 1
    > -#define num_online_cpus() cpumask_weight(cpu_online_mask)
    > -#define num_possible_cpus() cpumask_weight(cpu_possible_mask)
    > +
    > +#define num_online_cpus() (nr_online_cpus)
    > +#define num_possible_cpus() (nr_cpu_ids)

    nr_cpu_ids mean maximum cpu id of cpus. if cpu id are sparse, maximum id
    doesn't match number of cpus.



    > +
    > #define num_present_cpus() cpumask_weight(cpu_present_mask)
    > #define num_active_cpus() cpumask_weight(cpu_active_mask)
    > #define cpu_online(cpu) cpumask_test_cpu((cpu), cpu_online_mask)
    > diff --git a/kernel/cpu.c b/kernel/cpu.c
    > index 2060c6e..eed2169 100644
    > --- a/kernel/cpu.c
    > +++ b/kernel/cpu.c
    > @@ -622,6 +622,13 @@ static DECLARE_BITMAP(cpu_active_bits, CONFIG_NR_CPUS) __read_mostly;
    > const struct cpumask *const cpu_active_mask = to_cpumask(cpu_active_bits);
    > EXPORT_SYMBOL(cpu_active_mask);
    >
    > +#ifdef CONFIG_HOTPLUG_CPU
    > +int nr_online_cpus;
    > +#else
    > +int nr_online_cpus __read_mostly;
    > +#endif
    > +EXPORT_SYMBOL(nr_online_cpus);

    You can always mark this to __read_mostly. other cpu hotplug stuff do so.
    Because of, I guess, cpu hotplug developers don't think hotplugging is
    frequently event.


    > void set_cpu_possible(unsigned int cpu, bool possible)
    > {
    > if (possible)
    > @@ -644,6 +651,8 @@ void set_cpu_online(unsigned int cpu, bool online)
    > cpumask_set_cpu(cpu, to_cpumask(cpu_online_bits));
    > else
    > cpumask_clear_cpu(cpu, to_cpumask(cpu_online_bits));
    > +
    > + nr_online_cpus = cpumask_weight(cpu_online_mask);
    > }

    I like this change. :)





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