linux内核分析———SLAB原理及实现

struct kmem_cache {

struct array_cache *array[NR_CPUS];

unsigned int batchcount;//从本地高速缓存交换的对象的数量

unsigned int limit;//本地高速缓存中空闲对象的数量

unsigned int shared;//是否存在共享CPU高速缓存

unsigned int buffer_size;//对象长度+填充字节

u32 reciprocal_buffer_size;//倒数，加快计算



unsigned int flags;/*高速缓存永久性的标志，当前只CFLGS_OFF_SLAB*/

unsigned int num;/*封装在一个单独的slab中的对象数目*/

unsigned int gfporder;/*每个slab包含的页框数取2为底的对数*/



gfp_t gfpflags;/* e.g. GFP_DMA分配页框是传递给伙伴系统的标志*/

size_t colour; /* cache colouring range缓存的颜色个数free/aln*/



unsigned int colour_off;

/*slab的基本对齐偏移，为aln的整数倍,用来计算left_over*/



struct kmem_cache *slabp_cache;

//slab描述符放在外部时使用，其指向的高速缓存来存储描述符

unsigned int slab_size;//单个slab头的大小，包括SLAB和对象描述符

unsigned int dflags; /*描述高速缓存动态属性，目前没用*/



/*构造函数*/

void(*ctor)(struct kmem_cache *, void *);

const char *name;

struct list_head next;//高速缓存描述符双向链表指针



/*统计量*/

#if STATS

unsigned long num_active;

unsigned long num_allocations;

unsigned long high_mark;

unsigned long grown;

unsigned long reaped;

unsigned long errors;

unsigned long max_freeable;

unsignedlong node_allocs;

unsigned long node_frees;

unsigned long node_overflow;

atomic_t allochit;

atomic_t allocmiss;

atomic_t freehit;

atomic_t freemiss;

#endif

#if DEBUG

into bj_offset;//对象间的偏移

int obj_size;//对象本身的大小，

#endif

//存放的是所有节点对应的相关数据。每个节点拥有各自的数据；

struc tkmem_list3 *nodelists[MAX_NUMNODES];/

}

/*

* struct array_cache

*

*Purpose:

* - LIFO ordering, to hand out cache-warm objectsfrom _alloc

* - reduce the number of linked list operations

* - reduce spinlock operations

*

* The limit is stored in the per-cpu structure toreduce the data cache

* footprint.

*

*/

struct array_cache {

unsigned int avail;//可用对象数目

unsigned int limit;//可拥有的最大对象数目，和kmem_cache中一样

unsigned int batchcount;//同kmem_cache

unsigned int touched;//是否在收缩后被访问过

spinlock_t lock;

void *entry[]; /*伪数组，没有任何数据项，其后为释放的对象指针数组*/

};

/*

* The slab lists for all objects.

*/

struct kmem_list3 {

struct list_head slabs_partial; /* partial listfirst, better asm code */

struct list_head slabs_full;

struct list_head slabs_free;

unsigned long free_objects;//半空和全空链表中对象的个数

unsigned int free_limit;//所有slab上允许未使用的对象最大数目

unsigned int colour_next; /* 下一个slab的颜色*/

spinlock_t list_lock;

struct array_cache *shared; /* shared per node */

struct array_cache **alien; /* on other nodes */

unsigned long next_reap; /* 两次缓存收缩时的间隔，降低次数，提高性能*/

int free_touched; /* 0收缩1获取一个对象*/

};

struct slab {

struct list_head list;//SLAB所在的链表

unsigned long colouroff;//SLAB中第一个对象的偏移

void *s_mem; /* including colour offset 第一个对象的地址*/

unsigned int inuse; /* num of objs active in slab被使用的对象数目*/

kmem_bufctl_t free;//下一个空闲对象的下标

unsigned short nodeid;//用于寻址在高速缓存中kmem_list3的下标

};

/**

* kmem_cache_create - Create a cache.

* @name: A string which is used in /proc/slabinfo toidentify this cache.

* @size: The size of objects to be created in thiscache.

* @align: The required alignment for the objects.

* @flags: SLAB flags

* @ctor: A constructor for the objects.

*

* Returns a ptr to the cache on success, NULL onfailure.

* Cannot be calledwithin a int, but can be interrupted.

* The @ctor is run when new pages are allocated bythe cache.



struct kmem_cache *

kmem_cache_create (const char *name, size_t size,size_t align,unsigned long flags,

void (*ctor)(struct kmem_cache *, void *))

{

size_t left_over, slab_size, ralign;

struct kmem_cache *cachep = NULL, *pc;

/*参数有效性检查，名字有效性，对象长度比处理器字长还短，或者超过了允许分配的最大值，不能处在中断上下文，可能导致睡眠*/

if (!name || in_interrupt() || (size <BYTES_PER_WORD) ||

size > KMALLOC_MAX_SIZE) {

printk(KERN_ERR "%s: Early error in slab%s\n", __FUNCTION__,

name);

BUG();

}



/*获得锁*/

mutex_lock(&cache_chain_mutex);

....

/*

将大小舍入到处理器字长的倍数

*/

if (size & (BYTES_PER_WORD - 1)) {

size += (BYTES_PER_WORD - 1);

size &= ~(BYTES_PER_WORD - 1);

}



/* 计算对齐值*/



//如果设置了该标志，则用硬件缓存行

if (flags & SLAB_HWCACHE_ALIGN) {

ralign = cache_line_size();//获得硬件缓存行

//如果对象足够小，则将对齐值减半，，尽可能增加单行对象数目

while (size <= ralign )

ralign /= 2;

} else {//否则使用处理器字长

ralign = BYTES_PER_WORD;

}



/*体系结构强制最小值*/

if (ralign < ARCH_SLAB_MINALIGN) {

ralign = ARCH_SLAB_MINALIGN;

}

/*调用者强制对齐值*/

if (ralign < align) {

ralign = align;

}

/*计算出对齐值.*/

align = ralign;







/*从cache_cache缓存中分配一个kmem_cache新实例*/

cachep = kmem_cache_zalloc(&cache_cache,GFP_KERNEL);

//填充cachep成员

cachep->obj_size = size;//将填充后的对象赋值,





//设置SLAB头位置

//如果对象大小超过一页的1/8则放在外部

if ((size >= (PAGE_SIZE >> 3)) &&!slab_early_init)

flags |= CFLGS_OFF_SLAB;//设置将SLAB放在外部

size = ALIGN(size, align);//按对齐大小对齐



//计算缓存长度

//利用calculate_slab_order迭代来找到理想的slab长度，size指对象的长度

left_over = calculate_slab_order(cachep, size,align, flags);



if (!cachep->num) {//NUM指SLAB对象的数目

printk(KERN_ERR

"kmem_cache_create: couldn't createcache %s.\n", name);

kmem_cache_free(&cache_cache, cachep);

cachep = NULL;

goto oops;

}



//再次计算SLAB头存放位置

//计算slab头的大小=对象的数目x对象描述符的大小+slab描述符

slab_size = ALIGN(cachep->num *sizeof(kmem_bufctl_t)

+ sizeof(struct slab), align);



//如果有足够的空间，容纳SLAB头则将其放在SLAB上

if (flags & CFLGS_OFF_SLAB && left_over>= slab_size) {

flags &= ~CFLGS_OFF_SLAB;

left_over -= slab_size;

}

//如果标志仍然存在，则计算外部的slab头大小

if (flags & CFLGS_OFF_SLAB) {

/* 此处不用对齐了*/

slab_size =

cachep->num * sizeof(kmem_bufctl_t) +sizeof(struct slab);

}



//着色

cachep->colour_off =cache_line_size();//

/* Offset must be a multiple of the alignment. */

if (cachep->colour_off< align)

cachep->colour_off = align;

cachep->colour = left_over /cachep->colour_off;//获取颜色值

cachep->slab_size = slab_size;

cachep->flags = flags;

cachep->gfpflags = 0; //分配页框的标志

if (CONFIG_ZONE_DMA_FLAG && (flags &SLAB_CACHE_DMA))

cachep->gfpflags |= GFP_DMA;

cachep->buffer_size = size;

cachep->reciprocal_buffer_size =reciprocal_value(size);

//如果在SLAB头在外部，则找一个合适的缓存指向slabp_cache，从通用缓存中

if (flags & CFLGS_OFF_SLAB) {

cachep->slabp_cache= kmem_find_general_cachep(slab_size, 0u);

BUG_ON(ZERO_OR_NULL_PTR(cachep->slabp_cache));

}

cachep->ctor = ctor;

cachep->name = name;



//设置per-cpu缓存

if (setup_cpu_cache(cachep)){

__kmem_cache_destroy(cachep);

cachep = NULL;

goto oops;

}



/* 加入链表*/

list_add(&cachep->next, &cache_chain);

/*解锁*/

mutex_unlock(&cache_chain_mutex);

return cachep;

}

static inline void *____cache_alloc(struct kmem_cache *cachep,gfp_t flags)

{

void *objp;

struct array_cache *ac;



check_irq_off();



ac = cpu_cache_get(cachep);//获得高速缓存中CPU缓存

if (likely(ac->avail)) {//如果CPU缓存中还有空间，则从中分配

STATS_INC_ALLOCHIT(cachep);

ac->touched = 1;

objp = ac->entry[--ac->avail];

} else {//否则要填充CPU高速缓存了

STATS_INC_ALLOCMISS(cachep);

objp = cache_alloc_refill(cachep,flags);

}

return objp;

}

//填充CPU高速缓存

static void *cache_alloc_refill(structkmem_cache *cachep, gfp_t flags)

{

int batchcount;

struct kmem_list3 *l3;

struct array_cache *ac;

int node;

ac = cpu_cache_get(cachep);//获得高所缓存所在本地CPU缓存

retry:

batchcount = ac->batchcount;

if (!ac->touched && batchcount > BATCHREFILL_LIMIT){

/*如果不经常活动，则部分填充*/

batchcount = BATCHREFILL_LIMIT;//16

}

l3 = cachep->nodelists[node];//获得相应的kmem_list3结构体

...

/* 先考虑从共享本地CPU高速缓存*/

if (l3->shared && transfer_objects(ac, l3->shared,batchcount))

goto alloc_done;



while (batchcount > 0) {//老老实实的从本高速缓存分配

struct list_head *entry;

struct slab *slabp;

/* Get slab alloc is to come from. */

entry = l3->slabs_partial.next;//半满的链表

if (entry == &l3->slabs_partial) {//如果半空的都没了，找全空的

l3->free_touched = 1;

entry = l3->slabs_free.next;

if (entry == &l3->slabs_free)//全空的也没了，必须扩充了

cache_grow(cachep, flags | GFP_THISNODE, node, NULL);

}

//此时，已经找到了一个链表(半空或者全空)

slabp = list_entry(entry, struct slab, list);//找到一个slab

check_slabp(cachep, slabp);

check_spinlock_acquired(cachep);

while (slabp->inuse < cachep->num &&batchcount--)

{//循环从slab中分配对象

ac->entry[ac->avail++] =slab_get_obj(cachep, slabp,node);

}

check_slabp(cachep, slabp);



/*将slab放到合适的链中:*/

list_del(&slabp->list);

if (slabp->free == BUFCTL_END)//如果已经没有空闲对象了，则放到满链表中

list_add(&slabp->list, &l3->slabs_full);

else//否则放在半满链表

list_add(&slabp->list, &l3->slabs_partial);

}

...

ac->touched = 1;

return ac->entry[--ac->avail];

}

//按次序从SLAB中起初对象

static void *slab_get_obj(struct kmem_cache *cachep, struct slab*slabp,

int nodeid)

{

void *objp =index_to_obj(cachep, slabp, slabp->free);//找到要找的对象

kmem_bufctl_t next;

slabp->inuse++;//增加计数器

next =slab_bufctl(slabp)[slabp->free];

//获得slab_bufctl[slab->free]的值，为下一次锁定的空闲下标

slabp->free =next;//将锁定下标放到free中

return objp;

}

//增加新的SLAB

static int cache_grow(structkmem_cache *cachep, gfp_t flags, int nodeid, void *objp)

{

struct slab *slabp;

size_t offset;

gfp_t local_flags;

struct kmem_list3 *l3;

...

l3 = cachep->nodelists[nodeid];

...

/* 计算偏移量和下一个颜色.*/

offset = l3->colour_next;//计算下一个颜色

l3->colour_next++;//如果到了最大值则回0

if (l3->colour_next >= cachep->colour)

l3->colour_next = 0;

offset *= cachep->colour_off;//计算此SLAB的偏移



//从伙伴系统获得物理页

objp = kmem_getpages(cachep, local_flags, nodeid);

...

/* 如果slab头放在外部，则调用此函数分配函数*/

slabp = alloc_slabmgmt(cachep, objp, offset,

local_flags & ~GFP_CONSTRAINT_MASK, nodeid);

slabp->nodeid = nodeid;//在kmem_cache中数组的下标

//依次对每个物理页的lru.next=cache，lru.prev=slab

slab_map_pages(cachep, slabp, objp);

//调用各个对象的构造器函数，初始化新SLAB中的对象

cache_init_objs(cachep, slabp);

/* 将新的SLAB加入到全空链表中*/

list_add_tail(&slabp->list, &(l3->slabs_free));

STATS_INC_GROWN(cachep);

l3->free_objects += cachep->num;//更新空闲对象的数目

...

return 0;

}

//真正的处理函数

static inline void __cache_free(struct kmem_cache *cachep, void*objp)

{

struct array_cache *ac = cpu_cache_get(cachep);

...



if (likely(ac->avail < ac->limit)){//如果CPU高速缓存还有位子，则直接释放

ac->entry[ac->avail++] = objp;

return;

} else {//否则需要将部分对象FLUSH到SLAB中了

STATS_INC_FREEMISS(cachep);

cache_flusharray(cachep, ac);

ac->entry[ac->avail++] = objp;

}

}

//将部分CPU高速缓存FLUSH到SLAB中

static void cache_flusharray(struct kmem_cache *cachep, structarray_cache *ac)

{

int batchcount;

struct kmem_list3 *l3;

int node = numa_node_id();



batchcount = ac->batchcount;//指定数量

l3 = cachep->nodelists[node];

if (l3->shared) {//如果共享CPU缓存存在,则将共享缓存填满，然后返回

struct array_cache *shared_array = l3->shared;

int max = shared_array->limit - shared_array->avail;

if (max) {//

if (batchcount > max)

batchcount = max;

//这里只是拷贝，并没有移除

memcpy(&(shared_array->entry[shared_array->avail]),

ac->entry, sizeof(void *) * batchcount);

shared_array->avail += batchcount;

goto free_done;

}

}

//否则需要释放到SLAB中了

free_block(cachep,ac->entry, batchcount, node);

free_done:

//对CPU高速缓存进行移除操作

spin_unlock(&l3->list_lock);

ac->avail -= batchcount;

memmove(ac->entry, &(ac->entry[batchcount]),sizeof(void *)*ac->avail);

}

//将nr_objects个对象释放到SLAB中，objpp指CPU缓存数组

static void free_block(struct kmem_cache *cachep, void **objpp,int nr_objects, int node)

{

int i;

struct kmem_list3 *l3;



for (i = 0; i < nr_objects; i++) {//对每一个对象处理，先从头部处理，LIFO

void *objp = objpp[i];

struct slab *slabp;



slabp = virt_to_slab(objp);//获得SLAB描述符

l3 = cachep->nodelists[node];

list_del(&slabp->list);//将SLAB从原来的链表中删除

check_spinlock_acquired_node(cachep, node);

check_slabp(cachep, slabp);

slab_put_obj(cachep, slabp, objp,node);//将objp放到slab中，和slab_get_obj相反

STATS_DEC_ACTIVE(cachep);

l3->free_objects++;//增加高速缓存的可用对象数目

check_slabp(cachep, slabp);



/*将SLAB重新插入链表*/

if (slabp->inuse == 0) {//如果SLAB是全空的

if (l3->free_objects > l3->free_limit)

{//并且高速缓存空闲对象已经超出限制，则需要将SLAB返回给底层页框管理器

l3->free_objects -= cachep->num;

slab_destroy(cachep, slabp);

} else {//直接插入空闲链表

list_add(&slabp->list, &l3->slabs_free);

}

} else {//直接插入部分空闲链表

list_add_tail(&slabp->list, &l3->slabs_partial);

}

}

}

/* Size description struct for general caches. */

struct cache_sizes {

size_t cs_size;//对象大小

struct kmem_cache *cs_cachep;//对应的高速缓存

struct kmem_cache *cs_dmacachep;//对应的DMA访问缓存

};

//通用高速缓存在/kmalloc_sizes.h

struct cache_sizes malloc_sizes[] = {

#define CACHE(x) { .cs_size = (x) },

#include <linux/kmalloc_sizes.h>

CACHE(ULONG_MAX)

#undef CACHE

};

#if (PAGE_SIZE == 4096)

CACHE(32)

#endif

CACHE(64)

#if L1_CACHE_BYTES < 64

CACHE(96)

#endif

CACHE(128)

#if L1_CACHE_BYTES < 128

CACHE(192)

#endif

CACHE(256)

CACHE(512)

CACHE(1024)

CACHE(2048)

CACHE(4096)

CACHE(8192)

CACHE(16384)

CACHE(32768)

CACHE(65536)

CACHE(131072)

#if KMALLOC_MAX_SIZE >= 262144

CACHE(262144)

#endif

#if KMALLOC_MAX_SIZE >= 524288

CACHE(524288)

#endif

#if KMALLOC_MAX_SIZE >= 1048576

CACHE(1048576)

#endif

#if KMALLOC_MAX_SIZE >= 2097152

CACHE(2097152)

#endif

#if KMALLOC_MAX_SIZE >= 4194304

CACHE(4194304)

#endif

#if KMALLOC_MAX_SIZE >= 8388608

CACHE(8388608)

#endif

#if KMALLOC_MAX_SIZE >= 16777216

CACHE(16777216)

#endif

#if KMALLOC_MAX_SIZE >= 33554432

CACHE(33554432)

#endif

//分配函数

static inline void *kmalloc(size_t size, gfp_t flags)

{

if (__builtin_constant_p(size))

{//是否用常数指定所需的内存长度

int i = 0;

//找到合适大小的i值

...

//按类型进行分配

#ifdef CONFIG_ZONE_DMA

if (flags & GFP_DMA)

return kmem_cache_alloc(malloc_sizes[i].cs_dmacachep,

flags);

#endif

return kmem_cache_alloc(malloc_sizes[i].cs_cachep, flags);

}//不使用常数指定

return __kmalloc(size, flags);

}

//大小不用指定的分配

static __always_inline void *__do_kmalloc(size_t size, gfp_tflags, void *caller)

{

struct kmem_cache *cachep;

cachep = __find_general_cachep(size, flags);//找一个合适大小的高速缓存

if (unlikely(ZERO_OR_NULL_PTR(cachep)))

return cachep;

return __cache_alloc(cachep, flags, caller);//分配函数

}

//kmalloc对应的释放函数

void kfree(const void *objp)

{

struct kmem_cache *c;

unsigned long flags;

...

c =virt_to_cache(objp);//获得高速缓存

debug_check_no_locks_freed(objp, obj_size(c));

__cache_free(c, (void*)objp);//调用此函数完成实质性的分配

local_irq_restore(flags);

}