[内核内存] slab分配器4---kmem_cache_init_late函数源码详解

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slab系统初始化过程中，待所有cpu都完成初始化后，通过调用kmem_cache_init_late来函数完善cache_chain上每个struct kmem_cache实例的cpu缓存机制（包括cpu本地高速缓存和每个节点上的cpu共享缓存shared cache）

kmem_cache_init_late()|---list_for_each_entry(cachep, &slab_caches, list)//遍历slab_caches全局链表中每个struct kmem_cache实例enable_cpucache(cachep, GFP_NOWAIT)//对每个struct kmem_cache实例缓存机制进行完善|--->cache_random_seq_create()//初始化struct kmem_cache实例num成员|--->计算并设置struct kmem_cache实例的limit,batchcount,shared成员|--->do_tune_cpucache()//struct kmem_cache实例缓存机制的完善和其node成员数组中数组项的初始化|--->__do_tune_cpucache()|--->alloc_kmem_cache_cpus()//分配新的本地高速缓存区域（从Per_cpu area分配）|--->将kmem_cache实例的cpu_cache指向新分配的本地高速缓存区域（Per_CPU内存空间）|---for_each_online_cpu(cpu)//遍历每个cpufree_block()//释放对应cpu旧的本地缓存中缓存的slab obj到对应的slab链表中|--->free_percpu()//释放旧的Per_CPU变量，struct kmem_cache实例的高速缓存cpu_cache|--->setup_kmem_cache_nodes()//初始化node数组中每个kmem_cache_nod实例（共享缓存区域														 更新和相关成员初始化）|---for_each_online_node(node)//遍历每个内存节点setup_kmem_cache_node()//对kmem_cache_nod实例进初始化|--->alloc_arraycache()//为当前节点共享缓存区域分配空间|--->将kmem_cache_node实例的shared成员指向新分配的共享缓存空间|--->init_cache_node()//初始化kmem_cache_node实例的的3个slab链																	 表等

// mm/slab.c
/**1.完善全局链表slab_caches上所有struct kmem_cache实例的缓存机制:*	  根据slab obj大小，给本地cpu高速缓存和每个节点shared cache重新分配内存空间，并释放旧缓存缓存的*    slab obj（free_block）和旧缓存本身(kfree)）*2.初始化全局链表slab_caches上所有struct kmem_cache实例的node数组中每个struct kmem_cache_node实例的 *  相关数据（主要是每个内存节点中3种类型slab 链表的初始化）*/
void __init kmem_cache_init_late(void)
{struct kmem_cache *cachep;slab_state = UP;/* 6) resize the head arrays to their final sizes */mutex_lock(&slab_mutex);//遍历slab_caches上所有的slab cache描述符（struct kmem_cache结构体实例）list_for_each_entry(cachep, &slab_caches, list)/**完善当前slab cache描述符的cpu高速缓存机制，并对slab cache描述符的node成员中的每个struct kmem_cache_node*实例进行初始化操作.*1.对本地cpu高速缓存（cachep->cpu_cache）,通过slab obj的大小重新计算array_cache每个成员的*  值，然后为每个cpu重新分配一个array_cache实例（每cpu变量），并替换旧的arry_cache。*2.若是NUMA结构，完善cpu共享高速缓存机制（分配足够的共享array_cache实例并完成地址关联）.*3.为slab cache描述符每个节点对应的struct kmem_cache_node实例的相关成员进行初始化操作，主要是完成3个*  slab链表的初始化*/if (enable_cpucache(cachep, GFP_NOWAIT))BUG();mutex_unlock(&slab_mutex);/* Done! */slab_state = FULL;#ifdef CONFIG_NUMA/** Register a memory hotplug callback that initializes and frees* node.**/hotplug_memory_notifier(slab_memory_callback, SLAB_CALLBACK_PRI);
#endif/** The reap timers are started later, with a module init call: That part* of the kernel is not yet operational.*/
}   

/* Called with slab_mutex held always */
static int enable_cpucache(struct kmem_cache *cachep, gfp_t gfp)
{int err;int limit = 0;int shared = 0;int batchcount = 0;//err = cache_random_seq_create(cachep, cachep->num, gfp);if (err)goto end;//判断该slab cache描述符是否属于某一个memcg组，是的画，用该组root节点对应的数据进行初始化if (!is_root_cache(cachep)) {struct kmem_cache *root = memcg_root_cache(cachep);limit = root->limit;shared = root->shared;batchcount = root->batchcount;}//找到memcg组，直接完成了初始化设置，不许要根据obj大小进行成员值预估if (limit && shared && batchcount)goto skip_setup;/** The head array serves three purposes:* - create a LIFO ordering, i.e. return objects that are cache-warm* - reduce the number of spinlock operations.* - reduce the number of linked list operations on the slab and*   bufctl chains: array operations are cheaper.* The numbers are guessed, we should auto-tune as described by* Bonwick.*/// 根据对象(slab obj)的大小计算local cache中对象的数量 if (cachep->size > 131072)limit = 1;else if (cachep->size > PAGE_SIZE)limit = 8;else if (cachep->size > 1024)limit = 24;else if (cachep->size > 256)limit = 54;elselimit = 120;/** CPU bound tasks (e.g. network routing) can exhibit cpu bound* allocation behaviour: Most allocs on one cpu, most free operations* on another cpu. For these cases, an efficient object passing between* cpus is necessary. This is provided by a shared array. The array* replaces Bonwick's magazine layer.* On uniprocessor, it's functionally equivalent (but less efficient)* to a larger limit. Thus disabled by default.* 多核下设置共享本地缓存实例的个数（struct kmem_cache的shared成员），该slab描述符每个节点的共享缓存中最大* obj数量为cachep->shared*batchcount*/shared = 0;if (cachep->size <= PAGE_SIZE && num_possible_cpus() > 1)shared = 8;batchcount = (limit + 1) / 2;
skip_setup:/**根据前面计算出的limit, batchcount, shared值，为当前slab cache缓存更新本地cpu高速缓存和设置共享cpu高速缓存*/err = do_tune_cpucache(cachep, limit, batchcount, shared, gfp);
end:if (err)pr_err("enable_cpucache failed for %s, error %d\n",cachep->name, -err);return err;
}

/**(1)设置slab cache描述符的本地cpu高速缓存cpu_cache（每cpu变量，是个数组，数组中每个元素对应每个cpu）:*	 struct kmem_cache的cpu_caches数组中的每个成员需要更新成根据slab obj实际大小计算出来的新struct *   array_cache实例，同时将旧的固定大小的struct array_cache释放*(2)设置slab cache描述符的共享cpu高速缓存：* 		a.设置struct kmem_cache的shared值*		b.设置struct kmem_cache的node数组成员中每个struct kmem_cache_node对应节点上的cpu共享缓存数组.*		   就是跟给该slab cache缓存对应的每个节点分配足够多的struct arrary_cache实例*		c.设置struct kmem_cache的node数组成员中每个struct kmem_cache_node实例对应节点的3个slab链表		*/
static int do_tune_cpucache(struct kmem_cache *cachep, int limit,int batchcount, int shared, gfp_t gfp)
{int ret;struct kmem_cache *c;ret = __do_tune_cpucache(cachep, limit, batchcount, shared, gfp);if (slab_state < FULL)return ret;if ((ret < 0) || !is_root_cache(cachep))return ret;lockdep_assert_held(&slab_mutex);for_each_memcg_cache(c, cachep) {/* return value determined by the root cache only */__do_tune_cpucache(c, limit, batchcount, shared, gfp);}return ret;
}

static int __do_tune_cpucache(struct kmem_cache *cachep, int limit,int batchcount, int shared, gfp_t gfp)
{struct array_cache __percpu *cpu_cache, *prev;int cpu;// 根据limit, batchcount数值，为每个cpu构建新的array_cache实例并存储在cpu_cache数组cpu_cache = alloc_kmem_cache_cpus(cachep, limit, batchcount);if (!cpu_cache)return -ENOMEM;//旧的固定大小的array_cache实例数组prev = cachep->cpu_cache;//将根据slab obj大小计算出来的新本地cpu高速缓存赋值给slab cache描述符的cpu_cache成员cachep->cpu_cache = cpu_cache;//各个cpu上的每cpu变量数据同步（cachep->cpu_cache更新）kick_all_cpus_sync();check_irq_on();//更新该slab cache描述符中与本地cpu高速缓存有关的成员数据cachep->batchcount = batchcount;cachep->limit = limit;cachep->shared = shared;if (!prev)goto setup_node;/**此循环就是将旧的本地cpu高速缓存中缓存的slab obj释放给其slab系统.因为slab系统初始化时由于每个本地高*速缓存对应的avail为0，所以此处可被忽略。*/for_each_online_cpu(cpu) {LIST_HEAD(list);int node;struct kmem_cache_node *n;struct array_cache *ac = per_cpu_ptr(prev, cpu);node = cpu_to_mem(cpu);n = get_node(cachep, node);spin_lock_irq(&n->list_lock);free_block(cachep, ac->entry, ac->avail, node, &list);spin_unlock_irq(&n->list_lock);slabs_destroy(cachep, &list);}//将旧的cpu本地高速缓存这一Per_CPU变量释放（释放到每个cpu的Per_CPU area）free_percpu(prev);setup_node:return setup_kmem_cache_nodes(cachep, gfp);//分配lab cache描述每个节点对应的共享缓存区域，然后将每个节点对												 应的struct kmem_cache_node实例成员进行初始化。
}

// mm/slab.c
/*This initializes kmem_cache_node or resizes various caches for all nodes*/
static int setup_kmem_cache_nodes(struct kmem_cache *cachep, gfp_t gfp)
{......//遍历每个节点，并给每个节点的本地cpu高速缓存进行更新for_each_online_node(node) {ret = setup_kmem_cache_node(cachep, node, gfp, true);}......
}/**给slab cache描述符node数组成员中的每个struct kmem_cache_node实例进行初始化，主要工作：* 1.共享高速缓存shared成员进行内存空间分配（缓存的entry数组长度cachep->shared*cachep->batchcount）* 2.3种类型slab链表的初始化。*/
static int setup_kmem_cache_node(struct kmem_cache *cachep,int node, gfp_t gfp, bool force_change)
{int ret = -ENOMEM;struct kmem_cache_node *n;struct array_cache *old_shared = NULL;struct array_cache *new_shared = NULL;struct alien_cache **new_alien = NULL;LIST_HEAD(list);if (use_alien_caches) {new_alien = alloc_alien_cache(node, cachep->limit, gfp);if (!new_alien)goto fail;}//多cpu，slab cache描述符需要给每个内存节点分配一个共享cpu缓存区域if (cachep->shared) {/**每个节点分配的共享缓存区域最多能缓存cachep->shared * cachep->batchcount个slab obj，分配对应空间，并*将虚拟地址记录到new_shared上*/new_shared = alloc_arraycache(node,cachep->shared * cachep->batchcount, 0xbaadf00d, gfp);if (!new_shared)goto fail;}/**1.对本slab cache描述符node数组成员中每个struct kmem_cache_node实例的free_limit成员进行初始化（*   free_limit表示该slab cache描述在对应节点中空闲obj的数量上限，超过该值就会将一定数量的slab obj释放到伙*   伴系统的空闲链表中）*2.对本slab cache描述符node数组成员中每个struct kmem_cache_node实例的3个类型slab链表进行初始化。*/ret = init_cache_node(cachep, node, gfp);if (ret)goto fail;//获取本slab cache描述符cachep对应节点node的struct kmem_cache_node的描述符n = get_node(cachep, node);spin_lock_irq(&n->list_lock);/**释放slab cache描述符管理的每个内存节点旧的cpu共享高速缓存中缓存的slab obj,释放到对应的slab链表中，就是释放*n->shared->entry数组中每个数组项指向的slab obj对象.*/if (n->shared && force_change) {free_block(cachep, n->shared->entry,n->shared->avail, node, &list);n->shared->avail = 0;}/**将新新分配并初始化的共享cpu缓存空间赋值给slab描述符对应节点的struct kmem_cache_node实例的shared成*员*/if (!n->shared || force_change) {old_shared = n->shared;n->shared = new_shared;new_shared = NULL;}if (!n->alien) {n->alien = new_alien;new_alien = NULL;}spin_unlock_irq(&n->list_lock);slabs_destroy(cachep, &list);/** To protect lockless access to n->shared during irq disabled context.* If n->shared isn't NULL in irq disabled context, accessing to it is* guaranteed to be valid until irq is re-enabled, because it will be* freed after synchronize_sched().*/if (old_shared && force_change)synchronize_sched();
fail://收尾工作释放旧的struct array_cache(释放到对应slab系统的本地高速缓存中)kfree(old_shared);kfree(new_shared);free_alien_cache(new_alien);return ret;
}

ps：

1. 上面代码中free_block和kfree函数区别:
   1. free_block函数是把struct array_cache实例中的avail个entry数组指向的slab obj释放到对应的slab系统中（对应节点的所属的slab链表上）
   2. kfree函数基本同于free_block，但kfree将struct array_cache的slab obj释放到对应slab系统的每cpu高速缓存中.

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