Linux Kernel Architecture

Chapter3:MemoryManagement

kmalloc kmem_cache_alloc

__cache_alloc

__ __cache_alloc

cache_alloc_refill

cache_grow

Object in per-CPU Cache

Ja

Take object from cache Return object

Return object

Nein

Find object and take

it from the cache

Insufficient space in available slabs?

Figure 3-50: Code flow diagram forkmem_cache_alloc.

Selecting a Cached Object

____cache_alloccan check relatively easily if an object is in the per-CPU cache, as the following code

excerpt shows:

mm/slab.c

static inline void *____cache_alloc(kmem_cache_t *cachep, gfp_t flags)

{

ac = ac_data(cachep);

if (likely(ac->avail)) {

ac->touched = 1;

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

}

else {

objp = cache_alloc_refill(cachep, flags);

}

return objp;

cachepis a pointer to thekmem_cache_tinstance of the cache used. Theac_datamacro yields the asso-

ciatedarray_cacheinstance for the currently active CPU by returningcachep->array[smp_processor_

id()].

As the objects in memory immediately follow thearray_cacheinstance, the kernel can access them

easily with the help of the dummy array at the end of the structure without the explicit need for pointer

arithmetic. The object is removed from the cache by decrementingac->avail.

Refilling the Per-CPU Cache

The workload is heavier when there are no more objects in the per-CPU cache. The refill operations

needed in this situation are located incache_alloc_refill, which is invoked when the allocation cannot

be satisfied directly from the per-CPU cache.