Linux 内核 内存管理分区伙伴分配器 ⑦ ( z-＞watermark[WMARK_MIN] 最低水位计算 | min_free_kbytes 初始化 )

Posted 2022-04-26 韩曙亮

文章目录

• 一、z->watermark[WMARK_MIN] 最低水位计算
• 二、min_free_kbytes 初始化过程
• 三、最低水位计算过程
• 四、nr_free_buffer_pages 函数原型
• 五、nr_free_zone_pages 函数原型

一、z->watermark[WMARK_MIN] 最低水位计算

---

在 内存区域 的水位控制机制 中 , 在 内存区域 zone 结构体中的 watermark 成员 表示 " 页分配器 " 使用的 区域水线 ;

struct zone 
	/* Read-mostly fields */

	/* zone watermarks, access with *_wmark_pages(zone) macros */
	unsigned long watermark[NR_WMARK];

源码路径 : linux-4.12\\include\\linux\\mmzone.h#255

其中涉及到的 NR_WMARK 值为 $4$ , 定义在如下枚举数据结构中 ,

enum zone_watermarks 
	WMARK_MIN,
	WMARK_LOW,
	WMARK_HIGH,
	NR_WMARK
;

#define min_wmark_pages(z) (z->watermark[WMARK_MIN])
#define low_wmark_pages(z) (z->watermark[WMARK_LOW])
#define high_wmark_pages(z) (z->watermark[WMARK_HIGH])

源码路径 : linux-4.12\\include\\linux\\mmzone.h#255

其中 min_wmark_pages(z) 宏定义对应的 z->watermark[WMARK_MIN] 需要通过 min_free_kbytes 值进行计算 ,

min_free_kbytes 表示 " 系统保留的最低空闲内存数 " ,

二、min_free_kbytes 初始化过程

---

min_free_kbytes 值在 init_per_zone_wmark_min 函数中进行初始化 ,

lowmem_kbytes 用于统计 低内存 中超过 高水位 物理页的个数 ;

lowmem_kbytes 是 高水位线 之上的内存 ;

内存区域 的 整体 物理页个数 是 zone 结构体中的 managed_pages 成员 ;

下图是整个内存区域的图 ;

	lowmem_kbytes = nr_free_buffer_pages() * (PAGE_SIZE >> 10);
	new_min_free_kbytes = int_sqrt(lowmem_kbytes * 16);

min_free_kbytes 值的范围是 $128$ ~ $65535$ ;

	if (new_min_free_kbytes > user_min_free_kbytes) 
		min_free_kbytes = new_min_free_kbytes;
		if (min_free_kbytes < 128)
			min_free_kbytes = 128;
		if (min_free_kbytes > 65536)
			min_free_kbytes = 65536;
	 

init_per_zone_wmark_min 函数源码如下 :

/*
 * Initialise min_free_kbytes.
 *
 * For small machines we want it small (128k min).  For large machines
 * we want it large (64MB max).  But it is not linear, because network
 * bandwidth does not increase linearly with machine size.  We use
 *
 *	min_free_kbytes = 4 * sqrt(lowmem_kbytes), for better accuracy:
 *	min_free_kbytes = sqrt(lowmem_kbytes * 16)
 *
 * which yields
 *
 * 16MB:	512k
 * 32MB:	724k
 * 64MB:	1024k
 * 128MB:	1448k
 * 256MB:	2048k
 * 512MB:	2896k
 * 1024MB:	4096k
 * 2048MB:	5792k
 * 4096MB:	8192k
 * 8192MB:	11584k
 * 16384MB:	16384k
 */
int __meminit init_per_zone_wmark_min(void)

	unsigned long lowmem_kbytes;
	int new_min_free_kbytes;

	lowmem_kbytes = nr_free_buffer_pages() * (PAGE_SIZE >> 10);
	new_min_free_kbytes = int_sqrt(lowmem_kbytes * 16);

	if (new_min_free_kbytes > user_min_free_kbytes) 
		min_free_kbytes = new_min_free_kbytes;
		if (min_free_kbytes < 128)
			min_free_kbytes = 128;
		if (min_free_kbytes > 65536)
			min_free_kbytes = 65536;
	 else 
		pr_warn("min_free_kbytes is not updated to %d because user defined value %d is preferred\\n",
				new_min_free_kbytes, user_min_free_kbytes);
	
	setup_per_zone_wmarks();
	refresh_zone_stat_thresholds();
	setup_per_zone_lowmem_reserve();

#ifdef CONFIG_NUMA
	setup_min_unmapped_ratio();
	setup_min_slab_ratio();
#endif

	return 0;

源码路径 : linux-4.12\\mm\\page_alloc.c#6976

三、最低水位计算过程

---

先调用 linux-4.12\\mm\\page_alloc.c#4331 位置的 nr_free_buffer_pages 函数 ,

/**
 * nr_free_buffer_pages - count number of pages beyond high watermark
 *
 * nr_free_buffer_pages() counts the number of pages which are beyond the high
 * watermark within ZONE_DMA and ZONE_NORMAL.
 */
unsigned long nr_free_buffer_pages(void)

在该函数中调用了 linux-4.12\\mm\\page_alloc.c#4305 位置的 nr_free_zone_pages 函数 ;

/**
 * nr_free_zone_pages - count number of pages beyond high watermark
 * @offset: The zone index of the highest zone
 *
 * nr_free_zone_pages() counts the number of counts pages which are beyond the
 * high watermark within all zones at or below a given zone index.  For each
 * zone, the number of pages is calculated as:
 *
 *     nr_free_zone_pages = managed_pages - high_pages
 */
static unsigned long nr_free_zone_pages(int offset)

四、nr_free_buffer_pages 函数原型

---

/**
 * nr_free_buffer_pages - count number of pages beyond high watermark
 *
 * nr_free_buffer_pages() counts the number of pages which are beyond the high
 * watermark within ZONE_DMA and ZONE_NORMAL.
 */
unsigned long nr_free_buffer_pages(void)

	return nr_free_zone_pages(gfp_zone(GFP_USER));

源码路径 : linux-4.12\\mm\\page_alloc.c#4331

五、nr_free_zone_pages 函数原型

---

/**
 * nr_free_zone_pages - count number of pages beyond high watermark
 * @offset: The zone index of the highest zone
 *
 * nr_free_zone_pages() counts the number of counts pages which are beyond the
 * high watermark within all zones at or below a given zone index.  For each
 * zone, the number of pages is calculated as:
 *
 *     nr_free_zone_pages = managed_pages - high_pages
 */
static unsigned long nr_free_zone_pages(int offset)

	struct zoneref *z;
	struct zone *zone;

	/* Just pick one node, since fallback list is circular */
	unsigned long sum = 0;

	struct zonelist *zonelist = node_zonelist(numa_node_id(), GFP_KERNEL);

	for_each_zone_zonelist(zone, z, zonelist, offset) 
		unsigned long size = zone->managed_pages;
		unsigned long high = high_wmark_pages(zone);
		if (size > high)
			sum += size - high;
	

	return sum;

源码路径 : linux-4.12\\mm\\page_alloc.c#4305