linux-kernel - Re: [PATCH] Avoid mask based num_possible_cpus and num_online

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Date:	Wed, 18 Jan 2012 00:55:00 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Venkatesh Pallipadi <venki@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mike Travis <travis@....com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [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@...gle.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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