get_cpu_mask

 get_cpu_mask

/* cpu_bit_bitmap[0] is empty - so we can back into it */
#define MASK_DECLARE_1(x)    [x+1][0] = (1UL << (x))
#define MASK_DECLARE_2(x)    MASK_DECLARE_1(x), MASK_DECLARE_1(x+1)
#define MASK_DECLARE_4(x)    MASK_DECLARE_2(x), MASK_DECLARE_2(x+2)
#define MASK_DECLARE_8(x)    MASK_DECLARE_4(x), MASK_DECLARE_4(x+4)

const unsigned long cpu_bit_bitmap[BITS_PER_LONG+1][BITS_TO_LONGS(NR_CPUS)] = {

    MASK_DECLARE_8(0),    MASK_DECLARE_8(8),
    MASK_DECLARE_8(16),    MASK_DECLARE_8(24),
#if BITS_PER_LONG > 32
    MASK_DECLARE_8(32),    MASK_DECLARE_8(40),
    MASK_DECLARE_8(48),    MASK_DECLARE_8(56),
#endif
};

static inline const struct cpumask *get_cpu_mask(unsigned int cpu)
{
    const unsigned long *p = cpu_bit_bitmap[1 + cpu % BITS_PER_LONG];
    p -= cpu / BITS_PER_LONG;
    return to_cpumask(p);
}

#define to_cpumask(bitmap)                        \
    ((struct cpumask *)(1 ? (bitmap)                \
                : (void *)sizeof(__check_is_bitmap(bitmap))))

上面cpu_bit_bitmap unsigned long型二维数组元素初始化如下：

[0+1][0] = 1<<0, [1+1][0] = 1<<1,
[2+1][0] = 1<<2, [3+1][0] = 1<<3,
[4+1][0] = 1<<4, [5+1][0] = 1<<5,
[6+1][0] = 1<<6, [7+1][0] = 1<<7,
[8+1][0] = 1<<8, [9+1][0] = 1<<9,
[10+1][0] = 1<<10, [11+1][0] = 1<<11,
[12+1][0] = 1<<12, [13+1][0] = 1<<13,
[14+1][0] = 1<<14, [15+1][0] = 1<<15,
...

[62+1][0] = 1<<62, [63+1][0] = 1<<63,

 以NR_CPUS为4、cpu index为2为例，上述cpu_bit_bitmap[][]将是cpu_bit_bitmap[65][1]

p = cpu_bit_bitmap[3]

p -= 0

所以p指向cpu_bit_bitmap[3]，这个指向的数据是1ul<<2

此时cpumask_t类型是一个结构体，里面只包含一个unsigned long bits[1]的数组：

typedef struct cpumask { unsigned long bits[1]; } cpumask_t

将1ul << 2转成cpumask_t即得到这个cpu的cpumask

以NR_CPUS为65、cpu index为64为例，上述cpu_bit_bitmap[][]将是cpu_bit_bitmap[65][2]

p = cpu_bit_bitmap[2]

p -= 1

所以p指向cpu_bit_bitmap[2]的前一个位置，即是cpu_bit_bitmap[1][1]，将cpu_bit_bitmap[1][1] & cpu_bit_bitmap[2][0]这两个unsigned long型转成cpumask_t类型，此时cpumask_t类型是unsigned long bits[2]类型的struct：

typedef struct cpumask { unsigned long bits[2]; } cpumask_t

超过64个cpu的case这部分还需要再继续确认下...