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SubjectRe: [PATCH] Avoid mask based num_possible_cpus and num_online_cpus
Hi,

On 01/20/2012 01:31 AM, Venkatesh Pallipadi wrote:

> Does this look better? Will send separate patch to fix code
> using num_possible_cpus() when they actually need nr_cpu_ids.
>
>
>
> 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 | 10 ++++++++--
> kernel/cpu.c | 5 +++++
> kernel/smp.c | 4 ++++
> 3 files changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index 4f7a632..ac3113b 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -23,10 +23,14 @@ typedef struct cpumask { DECLARE_BITMAP(bits, NR_CPUS); } cpumask_t;
>
> #if NR_CPUS == 1
> #define nr_cpu_ids 1
> +#define nr_possible_cpus 1
> #else
> extern int nr_cpu_ids;
> +extern int nr_possible_cpus;
> #endif
>
> +extern int nr_online_cpus;
> +
> #ifdef CONFIG_CPUMASK_OFFSTACK
> /* Assuming NR_CPUS is huge, a runtime limit is more efficient. Also,
> * not all bits may be allocated. */
> @@ -81,8 +85,10 @@ extern const struct cpumask *const cpu_present_mask;
> extern const struct cpumask *const cpu_active_mask;
>
> #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_possible_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..f179baa 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -622,6 +622,9 @@ 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);
>
> +int nr_online_cpus __read_mostly;
> +EXPORT_SYMBOL(nr_online_cpus);
> +
> void set_cpu_possible(unsigned int cpu, bool possible)
> {
> if (possible)


Did you forget to add:

nr_possible_cpus = cpumask_weight(cpu_possible_mask);

inside set_cpu_possible() ?


> @@ -644,6 +647,8 @@ 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);
> }
>
> void set_cpu_active(unsigned int cpu, bool active)
> diff --git a/kernel/smp.c b/kernel/smp.c
> index db197d6..106e519 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -658,10 +658,14 @@ early_param("maxcpus", maxcpus);
> int nr_cpu_ids __read_mostly = NR_CPUS;
> EXPORT_SYMBOL(nr_cpu_ids);
>
> +int nr_possible_cpus __read_mostly = NR_CPUS;
> +EXPORT_SYMBOL(nr_possible_cpus);
> +
> /* An arch may set nr_cpu_ids earlier if needed, so this would be redundant */
> void __init setup_nr_cpu_ids(void)
> {
> nr_cpu_ids = find_last_bit(cpumask_bits(cpu_possible_mask),NR_CPUS) + 1;
> + nr_possible_cpus = cpumask_weight(cpu_possible_mask);
> }
>
> /* Called by boot processor to activate the rest. */


Regards,
Srivatsa S. Bhat



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