lkml.org 
[lkml]   [2012]   [Jan]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
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. :)





\
 
 \ /
  Last update: 2012-01-18 06:57    [W:0.935 / U:0.784 seconds]
©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site