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Subject[PATCH 6/7] Change dmapool free block management
Date
Use a list of free blocks within a page instead of using a bitmap.
Update documentation to reflect this. As well as being a slight
reduction in memory allocation, locked ops and lines of code, it speeds
up a transaction processing benchmark by 0.4%.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
---
mm/dmapool.c | 119 ++++++++++++++++++++++++++++-----------------------------
1 files changed, 58 insertions(+), 61 deletions(-)
diff --git a/mm/dmapool.c b/mm/dmapool.c
index e2ea454..72e7ece 100644
--- a/mm/dmapool.c
+++ b/mm/dmapool.c
@@ -17,7 +17,9 @@
* The current design of this allocator is fairly simple. The pool is
* represented by the 'struct dma_pool' which keeps a doubly-linked list of
* allocated pages. Each page in the page_list is split into blocks of at
- * least 'size' bytes.
+ * least 'size' bytes. Free blocks are tracked in an unsorted singly-linked
+ * list of free blocks within the page. Used blocks aren't tracked, but we
+ * keep a count of how many are currently allocated from each page.
*/

#include <linux/device.h>
@@ -38,7 +40,6 @@
struct dma_pool { /* the pool */
struct list_head page_list;
spinlock_t lock;
- size_t blocks_per_page;
size_t size;
struct device *dev;
size_t allocation;
@@ -51,8 +52,8 @@ struct dma_page { /* cacheable header for 'allocation' bytes */
struct list_head page_list;
void *vaddr;
dma_addr_t dma;
- unsigned in_use;
- unsigned long bitmap[0];
+ unsigned int in_use;
+ unsigned int offset;
};

#define POOL_TIMEOUT_JIFFIES ((100 /* msec */ * HZ) / 1000)
@@ -87,8 +88,8 @@ show_pools(struct device *dev, struct device_attribute *attr, char *buf)

/* per-pool info, no real statistics yet */
temp = scnprintf(next, size, "%-16s %4u %4Zu %4Zu %2u\n",
- pool->name,
- blocks, pages * pool->blocks_per_page,
+ pool->name, blocks,
+ pages * (pool->allocation / pool->size),
pool->size, pages);
size -= temp;
next += temp;
@@ -132,8 +133,11 @@ struct dma_pool *dma_pool_create(const char *name, struct device *dev,
return NULL;
}

- if (size == 0)
+ if (size == 0) {
return NULL;
+ } else if (size < 4) {
+ size = 4;
+ }

if ((size % align) != 0)
size = ALIGN(size, align);
@@ -160,7 +164,6 @@ struct dma_pool *dma_pool_create(const char *name, struct device *dev,
spin_lock_init(&retval->lock);
retval->size = size;
retval->allocation = allocation;
- retval->blocks_per_page = allocation / size;
init_waitqueue_head(&retval->waitq);

if (dev) {
@@ -186,28 +189,36 @@ struct dma_pool *dma_pool_create(const char *name, struct device *dev,
}
EXPORT_SYMBOL(dma_pool_create);

+static void pool_initialise_page(struct dma_pool *pool, struct dma_page *page)
+{
+ unsigned int offset = 0;
+
+ do {
+ unsigned int next = offset + pool->size;
+ if (unlikely((next + pool->size) >= pool->allocation))
+ next = pool->allocation;
+ *(int *)(page->vaddr + offset) = next;
+ offset = next;
+ } while (offset < pool->allocation);
+}
+
static struct dma_page *pool_alloc_page(struct dma_pool *pool, gfp_t mem_flags)
{
struct dma_page *page;
- int mapsize;
-
- mapsize = pool->blocks_per_page;
- mapsize = (mapsize + BITS_PER_LONG - 1) / BITS_PER_LONG;
- mapsize *= sizeof(long);

- page = kmalloc(mapsize + sizeof *page, mem_flags);
+ page = kmalloc(sizeof(*page), mem_flags);
if (!page)
return NULL;
- page->vaddr = dma_alloc_coherent(pool->dev,
- pool->allocation,
+ page->vaddr = dma_alloc_coherent(pool->dev, pool->allocation,
&page->dma, mem_flags);
if (page->vaddr) {
- memset(page->bitmap, 0xff, mapsize); /* bit set == free */
#ifdef CONFIG_DEBUG_SLAB
memset(page->vaddr, POOL_POISON_FREED, pool->allocation);
#endif
+ pool_initialise_page(pool, page);
list_add(&page->page_list, &pool->page_list);
page->in_use = 0;
+ page->offset = 0;
} else {
kfree(page);
page = NULL;
@@ -215,14 +226,9 @@ static struct dma_page *pool_alloc_page(struct dma_pool *pool, gfp_t mem_flags)
return page;
}

-static inline int is_page_busy(int blocks, unsigned long *bitmap)
+static inline int is_page_busy(struct dma_page *page)
{
- while (blocks > 0) {
- if (*bitmap++ != ~0UL)
- return 1;
- blocks -= BITS_PER_LONG;
- }
- return 0;
+ return page->in_use != 0;
}

static void pool_free_page(struct dma_pool *pool, struct dma_page *page)
@@ -257,7 +263,7 @@ void dma_pool_destroy(struct dma_pool *pool)
struct dma_page *page;
page = list_entry(pool->page_list.next,
struct dma_page, page_list);
- if (is_page_busy(pool->blocks_per_page, page->bitmap)) {
+ if (is_page_busy(page)) {
if (pool->dev)
dev_err(pool->dev,
"dma_pool_destroy %s, %p busy\n",
@@ -292,27 +298,14 @@ void *dma_pool_alloc(struct dma_pool *pool, gfp_t mem_flags,
{
unsigned long flags;
struct dma_page *page;
- int map, block;
size_t offset;
void *retval;

spin_lock_irqsave(&pool->lock, flags);
restart:
list_for_each_entry(page, &pool->page_list, page_list) {
- int i;
- /* only cachable accesses here ... */
- for (map = 0, i = 0;
- i < pool->blocks_per_page; i += BITS_PER_LONG, map++) {
- if (page->bitmap[map] == 0)
- continue;
- block = ffz(~page->bitmap[map]);
- if ((i + block) < pool->blocks_per_page) {
- clear_bit(block, &page->bitmap[map]);
- offset = (BITS_PER_LONG * map) + block;
- offset *= pool->size;
- goto ready;
- }
- }
+ if (page->offset < pool->allocation)
+ goto ready;
}
page = pool_alloc_page(pool, GFP_ATOMIC);
if (!page) {
@@ -333,10 +326,10 @@ void *dma_pool_alloc(struct dma_pool *pool, gfp_t mem_flags,
goto done;
}

- clear_bit(0, &page->bitmap[0]);
- offset = 0;
ready:
page->in_use++;
+ offset = page->offset;
+ page->offset = *(int *)(page->vaddr + offset);
retval = offset + page->vaddr;
*handle = offset + page->dma;
#ifdef CONFIG_DEBUG_SLAB
@@ -379,7 +372,7 @@ void dma_pool_free(struct dma_pool *pool, void *vaddr, dma_addr_t dma)
{
struct dma_page *page;
unsigned long flags;
- int map, block;
+ unsigned int offset;

page = pool_find_page(pool, dma);
if (!page) {
@@ -393,13 +386,9 @@ void dma_pool_free(struct dma_pool *pool, void *vaddr, dma_addr_t dma)
return;
}

- block = dma - page->dma;
- block /= pool->size;
- map = block / BITS_PER_LONG;
- block %= BITS_PER_LONG;
-
+ offset = vaddr - page->vaddr;
#ifdef CONFIG_DEBUG_SLAB
- if (((dma - page->dma) + (void *)page->vaddr) != vaddr) {
+ if ((dma - page->dma) != offset) {
if (pool->dev)
dev_err(pool->dev,
"dma_pool_free %s, %p (bad vaddr)/%Lx\n",
@@ -410,28 +399,36 @@ void dma_pool_free(struct dma_pool *pool, void *vaddr, dma_addr_t dma)
pool->name, vaddr, (unsigned long long)dma);
return;
}
- if (page->bitmap[map] & (1UL << block)) {
- if (pool->dev)
- dev_err(pool->dev,
- "dma_pool_free %s, dma %Lx already free\n",
- pool->name, (unsigned long long)dma);
- else
- printk(KERN_ERR
- "dma_pool_free %s, dma %Lx already free\n",
- pool->name, (unsigned long long)dma);
- return;
+ {
+ unsigned int chain = page->offset;
+ while (chain < pool->allocation) {
+ if (chain != offset) {
+ chain = *(int *)(page->vaddr + chain);
+ continue;
+ }
+ if (pool->dev)
+ dev_err(pool->dev, "dma_pool_free %s, dma %Lx "
+ "already free\n", pool->name,
+ (unsigned long long)dma);
+ else
+ printk(KERN_ERR "dma_pool_free %s, dma %Lx "
+ "already free\n", pool->name,
+ (unsigned long long)dma);
+ return;
+ }
}
memset(vaddr, POOL_POISON_FREED, pool->size);
#endif

spin_lock_irqsave(&pool->lock, flags);
page->in_use--;
- set_bit(block, &page->bitmap[map]);
+ *(int *)vaddr = page->offset;
+ page->offset = offset;
if (waitqueue_active(&pool->waitq))
wake_up_locked(&pool->waitq);
/*
* Resist a temptation to do
- * if (!is_page_busy(bpp, page->bitmap)) pool_free_page(pool, page);
+ * if (!is_page_busy(page)) pool_free_page(pool, page);
* Better have a few empty pages hang around.
*/
spin_unlock_irqrestore(&pool->lock, flags);
--
1.4.4.4

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