start_kernel——setup_nr_cpu_ids

kernel/smp.c

/* 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;
}

include/linux/cpumask.h

#if NR_CPUS == 1
#define nr_cpu_ids      1
#else
extern int nr_cpu_ids;
#endif

NR_CPUS不是1，所以编译extern int nr_cpu_ids;，我在源代码中没有找到nr_cpu_ids的定义，资料上说nr_cpu_ids全局变量被声明为__read_mostly属性。
nr_cpu_ids保存的是所有可处于联机状态的CPU总数。
nr_cpu_ids具有当前系统能具备的CPU数的信息，默认值为NR_CPUS值，NR_CPUS是编译时用户可设置的常量值。NR_CPUS并非当前系统内存在的CPU的数值，而是Linux内核能支持的最大CPU数的最大值。
UP(Uni-Processor)中是1，32位SMP中具有2~32的值，64位内核中具有2~64位的值。
kernel/cpu.c

#ifdef CONFIG_INIT_ALL_POSSIBLE
static DECLARE_BITMAP(cpu_possible_bits, CONFIG_NR_CPUS) __read_mostly
    = CPU_BITS_ALL;
#else
static DECLARE_BITMAP(cpu_possible_bits, CONFIG_NR_CPUS) __read_mostly;
#endif

没有定义CONFIG_INIT_ALL_POSSIBLE，所以编译

static DECLARE_BITMAP(cpu_possible_bits, CONFIG_NR_CPUS) __read_mostly;

DECLARE_BITMAP我们在http://blog.csdn.net/yin262/article/details/46774041已经分析过了，这句定义的最终结果是生车功能一个unsigned long 类型的数组，数组名为cpu_possible_bits，用位图来管理cpu个数，每一位对应一个cpu。

关于利用位图来管理cpu变量，还有以下：
include/linux/cpumask.h

/*
 * The following particular system cpumasks and operations manage
 * possible, present, active and online cpus.
 *
 *     cpu_possible_mask- has bit ‘cpu‘ set iff cpu is populatable
 *     cpu_present_mask - has bit ‘cpu‘ set iff cpu is populated
 *     cpu_online_mask  - has bit ‘cpu‘ set iff cpu available to scheduler
 *     cpu_active_mask  - has bit ‘cpu‘ set iff cpu available to migration
 *
 *  If !CONFIG_HOTPLUG_CPU, present == possible, and active == online.
 *
 *  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.  The cpu_present_mask is dynamic(*),
 *  representing which CPUs are currently plugged in.  And
 *  cpu_online_mask is the dynamic subset of cpu_present_mask,
 *  indicating those CPUs available for scheduling.
 *
 *  If HOTPLUG is enabled, then cpu_possible_mask is forced to have
 *  all NR_CPUS bits set, otherwise it is just the set of CPUs that
 *  ACPI reports present at boot.
 *
 *  If HOTPLUG is enabled, then cpu_present_mask varies dynamically,
 *  depending on what ACPI reports as currently plugged in, otherwise
 *  cpu_present_mask is just a copy of cpu_possible_mask.
 *
 *  (*) Well, cpu_present_mask is dynamic in the hotplug case.  If not
 *      hotplug, it‘s a copy of cpu_possible_mask, hence fixed at boot.
 *
 * Subtleties:
 * 1) UP arch‘s (NR_CPUS == 1, CONFIG_SMP not defined) hardcode
 *    assumption that their single CPU is online.  The UP
 *    cpu_{online,possible,present}_masks are placebos.  Changing them
 *    will have no useful affect on the following num_*_cpus()
 *    and cpu_*() macros in the UP case.  This ugliness is a UP
 *    optimization - don‘t waste any instructions or memory references
 *    asking if you‘re online or how many CPUs there are if there is
 *    only one CPU.
 */

extern const struct cpumask *const cpu_possible_mask;
extern const struct cpumask *const cpu_online_mask;
extern const struct cpumask *const cpu_present_mask;
extern const struct cpumask *const cpu_active_mask;

cpu_possible_mask：系统内可安装的最多CPU的位图
cpu_online_mask：系统内安装的CPU中，正在使用的CPU的位图
cpu_present_mask：系统内安装的CPU的位图
cpu_active_mask：处于联机状态且可以动的(migration)的CPU的位图

以上都是结构体指针类型，由相应的bitmap转化而来：
kernel/cpu.c

#ifdef CONFIG_INIT_ALL_POSSIBLE
static DECLARE_BITMAP(cpu_possible_bits, CONFIG_NR_CPUS) __read_mostly
    = CPU_BITS_ALL;
#else
static DECLARE_BITMAP(cpu_possible_bits, CONFIG_NR_CPUS) __read_mostly;
#endif
const struct cpumask *const cpu_possible_mask = to_cpumask(cpu_possible_bits);
EXPORT_SYMBOL(cpu_possible_mask);

static DECLARE_BITMAP(cpu_online_bits, CONFIG_NR_CPUS) __read_mostly;
const struct cpumask *const cpu_online_mask = to_cpumask(cpu_online_bits);
EXPORT_SYMBOL(cpu_online_mask);

static DECLARE_BITMAP(cpu_present_bits, CONFIG_NR_CPUS) __read_mostly;
const struct cpumask *const cpu_present_mask = to_cpumask(cpu_present_bits);
EXPORT_SYMBOL(cpu_present_mask);

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);

转换的过程，在之前的文章里也分析了。