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

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Date:	Fri, 20 Jan 2012 02:13:11 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Venkatesh Pallipadi <venki@...gle.com>
CC:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mike Travis <travis@....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

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@...gle.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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