Linux Slab分配器(三)--创建缓存
水平有限，描述不当之处还请之处，转载请注明出处http://blog.csdn.net/vanbreaker/article/details/7670272
创建新的缓存必须通过kmem_cache_create()函数来完成，原型如下
struct kmem_cache * kmem_cache_create (const char *name, size_t size, size_t align, unsigned long flags, void (*ctor)(void *))
- name:所创建的新缓存的名字
- size:缓存所分配对象的大小
- align:对象的对齐值
- flags:创建用的标识
- ctor:创建对象时的构造函数
kmem_cache_create()的实际工作就是为新的缓存申请缓存描述符，array_cache描述符和kmem_list3描述符，并根据接收的参数对这三个结构中的变量进行相应的初始化。新创建的缓存是空的，不包含slab。
struct kmem_cache *
kmem_cache_create (const char *name, size_t size, size_t align,
unsigned long flags, void (*ctor)(void *))
{
size_t left_over, slab_size, ralign;
struct kmem_cache *cachep = NULL, *pc;
gfp_t gfp;
/*
* Sanity checks... these are all serious usage bugs.
*/
/*做一些必要的检查，以下情况都是不合法的:
1.缓存名为空
2.处于中断环境中
3.缓存中的对象大小小于处理器的字长
4.缓存中的对象大小大于普通缓存的最大长度*/
if (!name || in_interrupt() || (size < BYTES_PER_WORD) ||
size > KMALLOC_MAX_SIZE) {
printk(KERN_ERR "%s: Early error in slab %s\n", __func__,
name);
BUG();
}
/*
* We use cache_chain_mutex to ensure a consistent view of
* cpu_online_mask as well. Please see cpuup_callback
*/
if (slab_is_available()) {
get_online_cpus();
mutex_lock(&cache_chain_mutex);
}
list_for_each_entry(pc, &cache_chain, next) {
char tmp;
int res;
/*
* This happens when the module gets unloaded and doesn't
* destroy its slab cache and no-one else reuses the vmalloc
* area of the module. Print a warning.
*/
res = probe_kernel_address(pc->name, tmp);
if (res) {
printk(KERN_ERR
"SLAB: cache with size %d has lost its name\n",
pc->buffer_size);
continue;
}
if (!strcmp(pc->name, name)) {
printk(KERN_ERR
"kmem_cache_create: duplicate cache %s\n", name);
dump_stack();
goto oops;
}
}
#if DEBUG
WARN_ON(strchr(name, ' ')); /* It confuses parsers */
#if FORCED_DEBUG
/*
* Enable redzoning and last user accounting, except for caches with
* large objects, if the increased size would increase the object size
* above the next power of two: caches with object sizes just above a
* power of two have a significant amount of internal fragmentation.
*/
if (size < 4096 || fls(size - 1) == fls(size-1 + REDZONE_ALIGN +
2 * sizeof(unsigned long long)))
flags |= SLAB_RED_ZONE | SLAB_STORE_USER;
if (!(flags & SLAB_DESTROY_BY_RCU))
flags |= SLAB_POISON;
#endif
if (flags & SLAB_DESTROY_BY_RCU)
BUG_ON(flags & SLAB_POISON);
#endif
/*
* Always checks flags, a caller might be expecting debug support which
* isn't available.
*/
BUG_ON(flags & ~CREATE_MASK);
/*
* Check that size is in terms of words. This is needed to avoid
* unaligned accesses for some archs when redzoning is used, and makes
* sure any on-slab bufctl's are also correctly aligned.
*/
/*如果缓存对象大小没有对齐到处理器字长，则对齐之*/
if (size & (BYTES_PER_WORD - 1)) {
size += (BYTES_PER_WORD - 1);
size &= ~(BYTES_PER_WORD - 1);
}
/* calculate the final buffer alignment: */
/* 1) arch recommendation: can be overridden for debug */
/*要求按照体系结构对齐*/
if (flags & SLAB_HWCACHE_ALIGN) {
/*
* Default alignment: as specified by the arch code. Except if
* an object is really small, then squeeze multiple objects into
* one cacheline.
*/
ralign = cache_line_size();/*对齐值取L1缓存行的大小*/
/*如果对象大小足够小，则不断压缩对齐值以保证能将足够多的对象装入一个缓存行*/
while (size <= ralign / 2)
ralign /= 2;
} else {
ralign = BYTES_PER_WORD; /*对齐值取处理器字长*/
}
/*
* Redzoning and user store require word alignment or possibly larger.
* Note this will be overridden by architecture or caller mandated
* alignment if either is greater than BYTES_PER_WORD.
*/
/*如果开启了DEBUG，则按需要进行相应的对齐*/
if (flags & SLAB_STORE_USER)
ralign = BYTES_PER_WORD;
if (flags & SLAB_RED_ZONE) {
ralign = REDZONE_ALIGN;
/* If redzoning, ensure that the second redzone is suitably
* aligned, by adjusting the object size accordingly. */
size += REDZONE_ALIGN - 1;
size &= ~(REDZONE_ALIGN - 1);
}
/* 2) arch mandated alignment */
if (ralign < ARCH_SLAB_MINALIGN) {
ralign = ARCH_SLAB_MINALIGN;
}
/* 3) caller mandated alignment */
if (ralign < align) {
ralign = align;
}
/* disable debug if necessary */
if (ralign > __alignof__(unsigned long long))
flags &= ~(SLAB_RED_ZONE | SLAB_STORE_USER);
/*
* 4) Store it.
*/
align = ralign;
if (slab_is_available())
gfp = GFP_KERNEL;
else
gfp = GFP_NOWAIT;
/* Get cache's description obj. */
/*从cache_cache中分配一个高速缓存描述符*/
cachep = kmem_cache_zalloc(&cache_cache, gfp);
if (!cachep)
goto oops;
#if DEBUG
cachep->obj_size = size;
/*
* Both debugging options require word-alignment which is calculated
* into align above.
*/
if (flags & SLAB_RED_ZONE) {
/* add space for red zone words */
cachep->obj_offset += sizeof(unsigned long long);
size += 2 * sizeof(unsigned long long);
}
if (flags & SLAB_STORE_USER) {
/* user store requires one word storage behind the end of
* the real object. But if the second red zone needs to be
* aligned to 64 bits, we must allow that much space.
*/
if (flags & SLAB_RED_ZONE)
size += REDZONE_ALIGN;
else
size += BYTES_PER_WORD;
}
#if FORCED_DEBUG && defined(CONFIG_DEBUG_PAGEALLOC)
if (size >= malloc_sizes[INDEX_L3 + 1].cs_size
&& cachep->obj_size > cache_line_size() && ALIGN(size, align) < PAGE_SIZE) {
cachep->obj_offset += PAGE_SIZE - ALIGN(size, align);
size = PAGE_SIZE;
}
#endif
#endif
/*
* Determine if the slab management is 'on' or 'off' slab.
* (bootstrapping cannot cope with offslab caches so don't do
* it too early on.)
*/
/*如果缓存对象的大小不小于页面大小的1/8并且不处于slab初始化阶段，
则选择将slab描述符放在slab外部以腾出更多的空间给对象*/
if ((size >= (PAGE_SIZE >> 3)) && !slab_early_init)
/*
* Size is large, assume best to place the slab management obj
* off-slab (should allow better packing of objs).
*/
flags |= CFLGS_OFF_SLAB;
/*将对象大小按之前确定的align对齐*/
size = ALIGN(size, align);
/*计算slab的对象数，分配给slab的页框阶数并返回slab的剩余空间，即碎片大小*/
left_over = calculate_slab_order(cachep, size, align, flags);
if (!cachep->num) {
printk(KERN_ERR
"kmem_cache_create: couldn't create cache %s.\n", name);
kmem_cache_free(&cache_cache, cachep);
cachep = NULL;
goto oops;
}
/*将slab管理区的大小按align进行对齐*/
slab_size = ALIGN(cachep->num * sizeof(kmem_bufctl_t)
+ sizeof(struct slab), align);
/*
* If the slab has been placed off-slab, and we have enough space then
* move it on-slab. This is at the expense of any extra colouring.
*/
/*如果之前确定将slab管理区放在slab外部，但是碎片空间大于slab管理区大小，
这时改变策略将slab管理区放在slab内部，这样可以节省外部空间，但是会牺牲
着色的颜色个数*/
if (flags & CFLGS_OFF_SLAB && left_over >= slab_size) {
flags &= ~CFLGS_OFF_SLAB;
left_over -= slab_size;
}
/*如果的确要将slab管理区放在外部，则不需按照该slab的对齐方式进行对齐了，
重新计算slab_size*/
if (flags & CFLGS_OFF_SLAB) {
/* really off slab. No need for manual alignment */
slab_size =
cachep->num * sizeof(kmem_bufctl_t) + sizeof(struct slab);
#ifdef CONFIG_PAGE_POISONING
/* If we're going to use the generic kernel_map_pages()
* poisoning, then it's going to smash the contents of
* the redzone and userword anyhow, so switch them off.
*/
if (size % PAGE_SIZE == 0 && flags & SLAB_POISON)
flags &= ~(SLAB_RED_ZONE | SLAB_STORE_USER);
#endif
}
/*着色偏移区L1缓存行的大小*/
cachep->colour_off = cache_line_size();
/* Offset must be a multiple of the alignment. */
if (cachep->colour_off < align)/*着色偏移小于align的话则要取对齐值*/
cachep->colour_off = align;
/*计算着色的颜色数目*/
cachep->colour = left_over / cachep->colour_off;
cachep->slab_size = slab_size;
cachep->flags = flags;
cachep->gfpflags = 0;
if (CONFIG_ZONE_DMA_FLAG && (flags & SLAB_CACHE_DMA))
cachep->gfpflags |= GFP_DMA;
cachep->buffer_size = size;
cachep->reciprocal_buffer_size = reciprocal_value(size);
if (flags & CFLGS_OFF_SLAB) {
cachep->slabp_cache = kmem_find_general_cachep(slab_size, 0u);
/*
* This is a possibility for one of the malloc_sizes caches.
* But since we go off slab only for object size greater than
* PAGE_SIZE/8, and malloc_sizes gets created in ascending order,
* this should not happen at all.
* But leave a BUG_ON for some lucky dude.
*/
BUG_ON(ZERO_OR_NULL_PTR(cachep->slabp_cache));
}
cachep->ctor = ctor;
cachep->name = name;
if (setup_cpu_cache(cachep, gfp)) {
__kmem_cache_destroy(cachep);
cachep = NULL;
goto oops;
}
/* cache setup completed, link it into the list */
/*将该高速缓存描述符添加进cache_chain*/
list_add(&cachep->next, &cache_chain);
oops:
if (!cachep && (flags & SLAB_PANIC))
panic("kmem_cache_create(): failed to create slab `%s'\n",
name);
if (slab_is_available()) {
mutex_unlock(&cache_chain_mutex);
put_online_cpus();
}
return cachep;
}
- 首先做参数有效性的检查
- 计算对齐值
- 分配一个缓存描述符
- 确定slab管理区(slab描述符+kmem_bufctl_t数组)的存储位置
- 调用calculate_slab_order()进行相关项的计算，包括分配给slab的页阶数，碎片大小，slab的对象数
- 计算着色偏移和可用的颜色数量
- 调用setup_cpu_cache()分配array_cache描述符和kmem_list3描述符并初始化相关变量
- 最后将缓存描述符插入cache_chain中
再来看看两个辅助函数calculate_slab_order()和setup_cpu_cache()
static size_t calculate_slab_order(struct kmem_cache *cachep,
size_t size, size_t align, unsigned long flags)
{
unsigned long offslab_limit;
size_t left_over = 0;
int gfporder;
for (gfporder = 0; gfporder <= KMALLOC_MAX_ORDER; gfporder++) {
unsigned int num;
size_t remainder;
/*根据gfporder计算对象数和剩余空间*/
cache_estimate(gfporder, size, align, flags, &remainder, &num);
if (!num)/*如果计算出来的对象数为0则要增大分配给slab的页框阶数再进行计算*/
continue;
if (flags & CFLGS_OFF_SLAB) {
/*
* Max number of objs-per-slab for caches which
* use off-slab slabs. Needed to avoid a possible
* looping condition in cache_grow().
*/
/*offslab_limit记录了在外部存储slab描述符时所允许的slab最大对象数*/
offslab_limit = size - sizeof(struct slab);
offslab_limit /= sizeof(kmem_bufctl_t);
/*如果前面计算出的对象数num要大于允许的最大对象数，则不合法*/
if (num > offslab_limit)
break;
}
/* Found something acceptable - save it away */
cachep->num = num;
cachep->gfporder = gfporder;
left_over = remainder;
/*
* A VFS-reclaimable slab tends to have most allocations
* as GFP_NOFS and we really don't want to have to be allocating
* higher-order pages when we are unable to shrink dcache.
*/
if (flags & SLAB_RECLAIM_ACCOUNT)
break;
/*
* Large number of objects is good, but very large slabs are
* currently bad for the gfp()s.
*/
if (gfporder >= slab_break_gfp_order)
break;
/*
* Acceptable internal fragmentation?
*/
if (left_over * 8 <= (PAGE_SIZE << gfporder))
break;
}
return left_over;
}
static void cache_estimate(unsigned long gfporder, size_t buffer_size,
size_t align, int flags, size_t *left_over,
unsigned int *num)
{
int nr_objs;
size_t mgmt_size;
size_t slab_size = PAGE_SIZE << gfporder;
/*
* The slab management structure can be either off the slab or
* on it. For the latter case, the memory allocated for a
* slab is used for:
*
* - The struct slab
* - One kmem_bufctl_t for each object
* - Padding to respect alignment of @align
* - @buffer_size bytes for each object
*
* If the slab management structure is off the slab, then the
* alignment will already be calculated into the size. Because
* the slabs are all pages aligned, the objects will be at the
* correct alignment when allocated.
*/
/*如果slab描述符存储在slab外部，则slab的对象数即为slab_size/buffer_size*/
if (flags & CFLGS_OFF_SLAB) {
mgmt_size = 0;
nr_objs = slab_size / buffer_size;
if (nr_objs > SLAB_LIMIT)
nr_objs = SLAB_LIMIT;
} else {/*否则先减去slab管理区的大小再进行计算*/
/*
* Ignore padding for the initial guess. The padding
* is at most @align-1 bytes, and @buffer_size is at
* least @align. In the worst case, this result will
* be one greater than the number of objects that fit
* into the memory allocation when taking the padding
* into account.
*/
nr_objs = (slab_size - sizeof(struct slab)) /
(buffer_size + sizeof(kmem_bufctl_t));
/*
* This calculated number will be either the right
* amount, or one greater than what we want.
*/
if (slab_mgmt_size(nr_objs, align) + nr_objs*buffer_size
> slab_size)
nr_objs--;
if (nr_objs > SLAB_LIMIT)
nr_objs = SLAB_LIMIT;
/*计算slab管理区的大小*/
mgmt_size = slab_mgmt_size(nr_objs, align);
}
/*保存slab对象数*/
*num = nr_objs;
/*计算并保存slab的剩余空间*/
*left_over = slab_size - nr_objs*buffer_size - mgmt_size;
}
在slab初始化完成后，也就是g_cpucache_up变量的值为FULL后，setup_cpu_cache()函数等价于setup_cpu_cache()-->enable_cpucache()
static int enable_cpucache(struct kmem_cache *cachep, gfp_t gfp)
{
int err;
int limit, shared;
/*
* 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.
*/
/*根据对象的大小来确定本地高速缓存中的空闲对象上限*/
if (cachep->buffer_size > 131072)
limit = 1;
else if (cachep->buffer_size > PAGE_SIZE)
limit = 8;
else if (cachep->buffer_size > 1024)
limit = 24;
else if (cachep->buffer_size > 256)
limit = 54;
else
limit = 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.
*/
shared = 0;
if (cachep->buffer_size <= PAGE_SIZE && num_possible_cpus() > 1)
shared = 8;
#if DEBUG
/*
* With debugging enabled, large batchcount lead to excessively long
* periods with disabled local interrupts. Limit the batchcount
*/
if (limit > 32)
limit = 32;
#endif
err = do_tune_cpucache(cachep, limit, (limit + 1) / 2, shared, gfp);
if (err)
printk(KERN_ERR "enable_cpucache failed for %s, error %d.\n",
cachep->name, -err);
return err;
}
static int do_tune_cpucache(struct kmem_cache *cachep, int limit,
int batchcount, int shared, gfp_t gfp)
{
struct ccupdate_struct *new;
int i;
/*申请一个ccupdate_struct*/
new = kzalloc(sizeof(*new), gfp);
if (!new)
return -ENOMEM;
/*为每个CPU申请array_cache和用来跟踪本地CPU空闲对象的指针数组*/
for_each_online_cpu(i) {
new->new[i] = alloc_arraycache(cpu_to_node(i), limit,
batchcount, gfp);
if (!new->new[i]) {
for (i--; i >= 0; i--)
kfree(new->new[i]);
kfree(new);
return -ENOMEM;
}
}
new->cachep = cachep;
/*将cachep和array_cache进关联*/
on_each_cpu(do_ccupdate_local, (void *)new, 1);
check_irq_on();
cachep->batchcount = batchcount;
cachep->limit = limit;
cachep->shared = shared;
for_each_online_cpu(i) {
struct array_cache *ccold = new->new[i];
if (!ccold)
continue;
spin_lock_irq(&cachep->nodelists[cpu_to_node(i)]->list_lock);
free_block(cachep, ccold->entry, ccold->avail, cpu_to_node(i));
spin_unlock_irq(&cachep->nodelists[cpu_to_node(i)]->list_lock);
kfree(ccold);
}
kfree(new);
/*申请kmem_list3*/
return alloc_kmemlist(cachep, gfp);
}