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Date: Tue, 7 Jun 2022 14:46:56 -0400
From: Tony Battersby <tonyb@...ernetics.com>
To: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc: iommu@...ts.linux-foundation.org, kernel-team@...com,
Matthew Wilcox <willy@...radead.org>,
Keith Busch <kbusch@...nel.org>,
Andy Shevchenko <andy.shevchenko@...il.com>,
Robin Murphy <robin.murphy@....com>,
Tony Lindgren <tony@...mide.com>
Subject: [PATCH v6 11/11] dmapool: improve scalability of dma_pool_free
dma_pool_free() scales poorly when the pool contains many pages because
pool_find_page() does a linear scan of all allocated pages. Improve its
scalability by replacing the linear scan with a red-black tree lookup.
In big O notation, this improves the algorithm from O(n) to O(log n).
Signed-off-by: Tony Battersby <tonyb@...ernetics.com>
---
Changes since v5:
pool_free_page() no longer exists.
Less churn in dma_pool_destroy().
Updated big O usage in description.
mm/dmapool.c | 114 ++++++++++++++++++++++++++++++++++++++++-----------
1 file changed, 90 insertions(+), 24 deletions(-)
diff --git a/mm/dmapool.c b/mm/dmapool.c
index fc9ae0683c20..31102a00fa7c 100644
--- a/mm/dmapool.c
+++ b/mm/dmapool.c
@@ -12,11 +12,12 @@
* Many older drivers still have their own code to do this.
*
* 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. 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.
+ * represented by the 'struct dma_pool' which keeps a red-black tree of all
+ * allocated pages, keyed by DMA address for fast lookup when freeing.
+ * Each page in the page_tree is split into blocks of at 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.
*
* The avail_page_list keeps track of pages that have one or more free blocks
* available to (re)allocate. Pages are moved in and out of avail_page_list
@@ -36,6 +37,7 @@
#include <linux/slab.h>
#include <linux/stat.h>
#include <linux/spinlock.h>
+#include <linux/rbtree.h>
#include <linux/string.h>
#include <linux/types.h>
#include <linux/wait.h>
@@ -45,7 +47,7 @@
#endif
struct dma_pool { /* the pool */
- struct list_head page_list;
+ struct rb_root page_tree;
struct list_head avail_page_list;
spinlock_t lock;
struct device *dev;
@@ -58,7 +60,7 @@ struct dma_pool { /* the pool */
};
struct dma_page { /* cacheable header for 'allocation' bytes */
- struct list_head page_list;
+ struct rb_node page_node;
struct list_head avail_page_link;
void *vaddr;
dma_addr_t dma;
@@ -69,11 +71,17 @@ struct dma_page { /* cacheable header for 'allocation' bytes */
static DEFINE_MUTEX(pools_lock);
static DEFINE_MUTEX(pools_reg_lock);
+static inline struct dma_page *rb_to_dma_page(struct rb_node *node)
+{
+ return rb_entry(node, struct dma_page, page_node);
+}
+
static ssize_t pools_show(struct device *dev, struct device_attribute *attr, char *buf)
{
int size;
struct dma_page *page;
struct dma_pool *pool;
+ struct rb_node *node;
size = sysfs_emit(buf, "poolinfo - 0.1\n");
@@ -83,7 +91,10 @@ static ssize_t pools_show(struct device *dev, struct device_attribute *attr, cha
size_t blocks = 0;
spin_lock_irq(&pool->lock);
- list_for_each_entry(page, &pool->page_list, page_list) {
+ for (node = rb_first(&pool->page_tree);
+ node;
+ node = rb_next(node)) {
+ page = rb_to_dma_page(node);
pages++;
blocks += page->in_use;
}
@@ -160,7 +171,7 @@ struct dma_pool *dma_pool_create(const char *name, struct device *dev,
retval->dev = dev;
- INIT_LIST_HEAD(&retval->page_list);
+ retval->page_tree = RB_ROOT;
INIT_LIST_HEAD(&retval->avail_page_list);
spin_lock_init(&retval->lock);
retval->size = size;
@@ -204,6 +215,63 @@ struct dma_pool *dma_pool_create(const char *name, struct device *dev,
}
EXPORT_SYMBOL(dma_pool_create);
+/*
+ * Find the dma_page that manages the given DMA address.
+ */
+static struct dma_page *pool_find_page(struct dma_pool *pool, dma_addr_t dma)
+{
+ struct rb_node *node = pool->page_tree.rb_node;
+
+ while (node) {
+ struct dma_page *page = rb_to_dma_page(node);
+
+ if (dma < page->dma)
+ node = node->rb_left;
+ else if ((dma - page->dma) >= pool->allocation)
+ node = node->rb_right;
+ else
+ return page;
+ }
+ return NULL;
+}
+
+/*
+ * Insert a dma_page into the page_tree.
+ */
+static int pool_insert_page(struct dma_pool *pool, struct dma_page *new_page)
+{
+ dma_addr_t dma = new_page->dma;
+ struct rb_node **node = &(pool->page_tree.rb_node), *parent = NULL;
+
+ while (*node) {
+ struct dma_page *this_page = rb_to_dma_page(*node);
+
+ parent = *node;
+ if (dma < this_page->dma)
+ node = &((*node)->rb_left);
+ else if (likely((dma - this_page->dma) >= pool->allocation))
+ node = &((*node)->rb_right);
+ else {
+ /*
+ * A page that overlaps the new DMA range is already
+ * present in the tree. This should not happen.
+ */
+ WARN(1,
+ "%s: %s: DMA address overlap: old %pad new %pad len %u\n",
+ dev_name(pool->dev),
+ pool->name, &this_page->dma, &dma,
+ pool->allocation);
+ return -1;
+ }
+ }
+
+ /* Add new node and rebalance tree. */
+ rb_link_node(&new_page->page_node, parent, node);
+ rb_insert_color(&new_page->page_node, &pool->page_tree);
+
+ return 0;
+}
+
static void pool_initialise_page(struct dma_pool *pool, struct dma_page *page)
{
unsigned int offset = 0;
@@ -274,7 +342,10 @@ void dma_pool_destroy(struct dma_pool *pool)
device_remove_file(pool->dev, &dev_attr_pools);
mutex_unlock(&pools_reg_lock);
- list_for_each_entry_safe(page, tmp, &pool->page_list, page_list) {
+ rbtree_postorder_for_each_entry_safe(page,
+ tmp,
+ &pool->page_tree,
+ page_node) {
void *vaddr = page->vaddr;
if (is_page_busy(page)) {
@@ -333,7 +404,15 @@ void *dma_pool_alloc(struct dma_pool *pool, gfp_t mem_flags,
spin_lock_irqsave(&pool->lock, flags);
- list_add(&page->page_list, &pool->page_list);
+ if (unlikely(pool_insert_page(pool, page))) {
+ /*
+ * This should not happen, so something must have gone horribly
+ * wrong. Instead of crashing, intentionally leak the memory
+ * and make for the exit.
+ */
+ spin_unlock_irqrestore(&pool->lock, flags);
+ return NULL;
+ }
list_add(&page->avail_page_link, &pool->avail_page_list);
ready:
page->in_use++;
@@ -375,19 +454,6 @@ void *dma_pool_alloc(struct dma_pool *pool, gfp_t mem_flags,
}
EXPORT_SYMBOL(dma_pool_alloc);
-static struct dma_page *pool_find_page(struct dma_pool *pool, dma_addr_t dma)
-{
- struct dma_page *page;
-
- list_for_each_entry(page, &pool->page_list, page_list) {
- if (dma < page->dma)
- continue;
- if ((dma - page->dma) < pool->allocation)
- return page;
- }
- return NULL;
-}
-
/**
* dma_pool_free - put block back into dma pool
* @pool: the dma pool holding the block
--
2.25.1
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