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slab: change the management method of free objects of the slab
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Current free objects management method of the slab is weird, because
it touch random position of the array of kmem_bufctl_t when we try to
get free object. See following example.

struct slab's free = 6
kmem_bufctl_t array: 1 END 5 7 0 4 3 2

To get free objects, we access this array with following pattern.
6 -> 3 -> 7 -> 2 -> 5 -> 4 -> 0 -> 1 -> END

If we have many objects, this array would be larger and be not in the same
cache line. It is not good for performance.

We can do same thing through more easy way, like as the stack.
Only thing we have to do is to maintain stack top to free object. I use
free field of struct slab for this purpose. After that, if we need to get
an object, we can get it at stack top and manipulate top pointer.
That's all. This method already used in array_cache management.
Following is an access pattern when we use this method.

struct slab's free = 0
kmem_bufctl_t array: 6 3 7 2 5 4 0 1

To get free objects, we access this array with following pattern.
0 -> 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7

This may help cache line footprint if slab has many objects, and,
in addition, this makes code much much simpler.

Acked-by: Andi Kleen <[email protected]>
Signed-off-by: Joonsoo Kim <[email protected]>
Signed-off-by: Pekka Enberg <[email protected]>
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@JoonsooKim @penberg
JoonsooKim authored and penberg committed Oct 24, 2013
1 parent a57a498 commit b1cb098
Showing 1 changed file with 30 additions and 64 deletions.
94 changes: 30 additions & 64 deletions mm/slab.c
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Original file line number Diff line number Diff line change
Expand Up @@ -183,9 +183,6 @@ static bool pfmemalloc_active __read_mostly;
*/

typedef unsigned int kmem_bufctl_t;
#define BUFCTL_END (((kmem_bufctl_t)(~0U))-0)
#define BUFCTL_FREE (((kmem_bufctl_t)(~0U))-1)
#define BUFCTL_ACTIVE (((kmem_bufctl_t)(~0U))-2)
#define SLAB_LIMIT (((kmem_bufctl_t)(~0U))-3)

/*
Expand Down Expand Up @@ -2653,9 +2650,8 @@ static void cache_init_objs(struct kmem_cache *cachep,
if (cachep->ctor)
cachep->ctor(objp);
#endif
slab_bufctl(slabp)[i] = i + 1;
slab_bufctl(slabp)[i] = i;
}
slab_bufctl(slabp)[i - 1] = BUFCTL_END;
}

static void kmem_flagcheck(struct kmem_cache *cachep, gfp_t flags)
Expand All @@ -2671,16 +2667,14 @@ static void kmem_flagcheck(struct kmem_cache *cachep, gfp_t flags)
static void *slab_get_obj(struct kmem_cache *cachep, struct slab *slabp,
int nodeid)
{
void *objp = index_to_obj(cachep, slabp, slabp->free);
kmem_bufctl_t next;
void *objp;

slabp->inuse++;
next = slab_bufctl(slabp)[slabp->free];
objp = index_to_obj(cachep, slabp, slab_bufctl(slabp)[slabp->free]);
#if DEBUG
slab_bufctl(slabp)[slabp->free] = BUFCTL_FREE;
WARN_ON(page_to_nid(virt_to_page(objp)) != nodeid);
#endif
slabp->free = next;
slabp->free++;

return objp;
}
Expand All @@ -2689,19 +2683,23 @@ static void slab_put_obj(struct kmem_cache *cachep, struct slab *slabp,
void *objp, int nodeid)
{
unsigned int objnr = obj_to_index(cachep, slabp, objp);

#if DEBUG
kmem_bufctl_t i;

/* Verify that the slab belongs to the intended node */
WARN_ON(page_to_nid(virt_to_page(objp)) != nodeid);

if (slab_bufctl(slabp)[objnr] + 1 <= SLAB_LIMIT + 1) {
printk(KERN_ERR "slab: double free detected in cache "
"'%s', objp %p\n", cachep->name, objp);
BUG();
/* Verify double free bug */
for (i = slabp->free; i < cachep->num; i++) {
if (slab_bufctl(slabp)[i] == objnr) {
printk(KERN_ERR "slab: double free detected in cache "
"'%s', objp %p\n", cachep->name, objp);
BUG();
}
}
#endif
slab_bufctl(slabp)[objnr] = slabp->free;
slabp->free = objnr;
slabp->free--;
slab_bufctl(slabp)[slabp->free] = objnr;
slabp->inuse--;
}

Expand Down Expand Up @@ -2862,9 +2860,6 @@ static void *cache_free_debugcheck(struct kmem_cache *cachep, void *objp,
BUG_ON(objnr >= cachep->num);
BUG_ON(objp != index_to_obj(cachep, slabp, objnr));

#ifdef CONFIG_DEBUG_SLAB_LEAK
slab_bufctl(slabp)[objnr] = BUFCTL_FREE;
#endif
if (cachep->flags & SLAB_POISON) {
#ifdef CONFIG_DEBUG_PAGEALLOC
if ((cachep->size % PAGE_SIZE)==0 && OFF_SLAB(cachep)) {
Expand All @@ -2881,33 +2876,9 @@ static void *cache_free_debugcheck(struct kmem_cache *cachep, void *objp,
return objp;
}

static void check_slabp(struct kmem_cache *cachep, struct slab *slabp)
{
kmem_bufctl_t i;
int entries = 0;

/* Check slab's freelist to see if this obj is there. */
for (i = slabp->free; i != BUFCTL_END; i = slab_bufctl(slabp)[i]) {
entries++;
if (entries > cachep->num || i >= cachep->num)
goto bad;
}
if (entries != cachep->num - slabp->inuse) {
bad:
printk(KERN_ERR "slab: Internal list corruption detected in "
"cache '%s'(%d), slabp %p(%d). Tainted(%s). Hexdump:\n",
cachep->name, cachep->num, slabp, slabp->inuse,
print_tainted());
print_hex_dump(KERN_ERR, "", DUMP_PREFIX_OFFSET, 16, 1, slabp,
sizeof(*slabp) + cachep->num * sizeof(kmem_bufctl_t),
1);
BUG();
}
}
#else
#define kfree_debugcheck(x) do { } while(0)
#define cache_free_debugcheck(x,objp,z) (objp)
#define check_slabp(x,y) do { } while(0)
#endif

static void *cache_alloc_refill(struct kmem_cache *cachep, gfp_t flags,
Expand Down Expand Up @@ -2957,7 +2928,6 @@ static void *cache_alloc_refill(struct kmem_cache *cachep, gfp_t flags,
}

slabp = list_entry(entry, struct slab, list);
check_slabp(cachep, slabp);
check_spinlock_acquired(cachep);

/*
Expand All @@ -2975,11 +2945,10 @@ static void *cache_alloc_refill(struct kmem_cache *cachep, gfp_t flags,
ac_put_obj(cachep, ac, slab_get_obj(cachep, slabp,
node));
}
check_slabp(cachep, slabp);

/* move slabp to correct slabp list: */
list_del(&slabp->list);
if (slabp->free == BUFCTL_END)
if (slabp->free == cachep->num)
list_add(&slabp->list, &n->slabs_full);
else
list_add(&slabp->list, &n->slabs_partial);
Expand Down Expand Up @@ -3054,16 +3023,6 @@ static void *cache_alloc_debugcheck_after(struct kmem_cache *cachep,
*dbg_redzone1(cachep, objp) = RED_ACTIVE;
*dbg_redzone2(cachep, objp) = RED_ACTIVE;
}
#ifdef CONFIG_DEBUG_SLAB_LEAK
{
struct slab *slabp;
unsigned objnr;

slabp = virt_to_slab(objp);
objnr = (unsigned)(objp - slabp->s_mem) / cachep->size;
slab_bufctl(slabp)[objnr] = BUFCTL_ACTIVE;
}
#endif
objp += obj_offset(cachep);
if (cachep->ctor && cachep->flags & SLAB_POISON)
cachep->ctor(objp);
Expand Down Expand Up @@ -3269,7 +3228,6 @@ static void *____cache_alloc_node(struct kmem_cache *cachep, gfp_t flags,

slabp = list_entry(entry, struct slab, list);
check_spinlock_acquired_node(cachep, nodeid);
check_slabp(cachep, slabp);

STATS_INC_NODEALLOCS(cachep);
STATS_INC_ACTIVE(cachep);
Expand All @@ -3278,12 +3236,11 @@ static void *____cache_alloc_node(struct kmem_cache *cachep, gfp_t flags,
BUG_ON(slabp->inuse == cachep->num);

obj = slab_get_obj(cachep, slabp, nodeid);
check_slabp(cachep, slabp);
n->free_objects--;
/* move slabp to correct slabp list: */
list_del(&slabp->list);

if (slabp->free == BUFCTL_END)
if (slabp->free == cachep->num)
list_add(&slabp->list, &n->slabs_full);
else
list_add(&slabp->list, &n->slabs_partial);
Expand Down Expand Up @@ -3445,11 +3402,9 @@ static void free_block(struct kmem_cache *cachep, void **objpp, int nr_objects,
n = cachep->node[node];
list_del(&slabp->list);
check_spinlock_acquired_node(cachep, node);
check_slabp(cachep, slabp);
slab_put_obj(cachep, slabp, objp, node);
STATS_DEC_ACTIVE(cachep);
n->free_objects++;
check_slabp(cachep, slabp);

/* fixup slab chains */
if (slabp->inuse == 0) {
Expand Down Expand Up @@ -4308,12 +4263,23 @@ static inline int add_caller(unsigned long *n, unsigned long v)
static void handle_slab(unsigned long *n, struct kmem_cache *c, struct slab *s)
{
void *p;
int i;
int i, j;

if (n[0] == n[1])
return;
for (i = 0, p = s->s_mem; i < c->num; i++, p += c->size) {
if (slab_bufctl(s)[i] != BUFCTL_ACTIVE)
bool active = true;

for (j = s->free; j < c->num; j++) {
/* Skip freed item */
if (slab_bufctl(s)[j] == i) {
active = false;
break;
}
}
if (!active)
continue;

if (!add_caller(n, (unsigned long)*dbg_userword(c, p)))
return;
}
Expand Down

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