1 方法1：使用getnstimeofday64方法

此处以获取内核中mm/page_alloc.c的__alloc_pages_direct_reclaim函数中的__perform_reclaim函数的执行之间为例（内核版本4.14.216），代码如下：

/* The really slow allocator path where we enter direct reclaim */
static inline struct page *
__alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
                             unsigned int alloc_flags, const struct alloc_context *ac,
                             unsigned long *did_some_progress)
{
	struct page *page = NULL;
    bool drained = false;
    struct timespec64 start, end;
	
	getnstimeofday64(&start);
    *did_some_progress = __perform_reclaim(gfp_mask, order, ac);
    getnstimeofday64(&end);
    printk("func : %s-> %s, time spent = %ld s, %ld ns\n", \
    	__func__, "__perform_reclaim", (end.tv_sec - start.tv_sec),\
    	(end.tv_nsec - start.tv_nsec));
   
   //... the rest of the code
}

2 方法2：使用ktime_get_real_ns方法

此处仍是以获取内核中mm/page_alloc.c的__alloc_pages_direct_reclaim函数中的__perform_reclaim函数的执行之间为例（内核版本4.14.216），代码如下：

/* The really slow allocator path where we enter direct reclaim */
static inline struct page *
__alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
                             unsigned int alloc_flags, const struct alloc_context *ac,
                             unsigned long *did_some_progress)
{
	struct page *page = NULL;
    bool drained = false;
    u64 start_time, end_time;
	
	start_time = ktime_get_real_ns();
    *did_some_progress = __perform_reclaim(gfp_mask, order, ac);
    end_time = ktime_get_real_ns();
    printk("func : %s-> %s, time spent = %ld ns\n", \
    	__func__, "__perform_reclaim", (end_time - start_time));
   
   //... the rest of the code
}

相关参考：
Linux latest timekeeping