Re: [PATCH v2 1/2] drm/buddy: Optimize free block management with RB tree

[Arunpravin Paneer Selvam]

On 8/20/2025 3:44 PM, Matthew Auld wrote:

On 20/08/2025 08:56, Arunpravin Paneer Selvam wrote:

Hi Matthew,

On 8/14/2025 4:15 PM, Matthew Auld wrote:

On 24/07/2025 11:46, Arunpravin Paneer Selvam wrote:

Replace the freelist (O(n)) used for free block management with a
red-black tree, providing more efficient O(log n) search, insert,
and delete operations. This improves scalability and performance
when managing large numbers of free blocks per order (e.g., hundreds
or thousands).

In the VK-CTS memory stress subtest, the buddy manager merges
fragmented memory and inserts freed blocks into the freelist. Since
freelist insertion is O(n), this becomes a bottleneck as fragmentation

increases. Benchmarking shows list_insert_sorted() consumes ~52.69% CPU

with the freelist, compared to just 0.03% with the RB tree
(rbtree_insert.isra.0), despite performing the same sorted insert.

This also improves performance in heavily fragmented workloads,
such as games or graphics tests that stress memory.

Neat. Also please Cc intel-...@lists.freedesktop.org and intel- x...@lists.freedesktop.org on the next revision so our CI can pick this up.

Sure, I will add on v3.

Signed-off-by: Arunpravin Paneer Selvam <arunpravin.paneersel...@amd.com>

---

  drivers/gpu/drm/drm_buddy.c | 141 +++++++++++++++++++++++-------------

  include/drm/drm_buddy.h     |  39 +++++++++-
  2 files changed, 128 insertions(+), 52 deletions(-)

diff --git a/drivers/gpu/drm/drm_buddy.c b/drivers/gpu/drm/drm_buddy.c
index a1e652b7631d..19e9773b41be 100644
--- a/drivers/gpu/drm/drm_buddy.c
+++ b/drivers/gpu/drm/drm_buddy.c

@@ -31,6 +31,8 @@ static struct drm_buddy_block *drm_block_alloc(struct drm_buddy *mm,

      block->header |= order;
      block->parent = parent;
  +    RB_CLEAR_NODE(&block->rb);
+
      BUG_ON(block->header & DRM_BUDDY_HEADER_UNUSED);
      return block;
  }
@@ -41,23 +43,53 @@ static void drm_block_free(struct drm_buddy *mm,
      kmem_cache_free(slab_blocks, block);
  }
  -static void list_insert_sorted(struct drm_buddy *mm,
-                   struct drm_buddy_block *block)
+static void rbtree_insert(struct drm_buddy *mm,
+              struct drm_buddy_block *block)
  {

+    struct rb_root *root = &mm- >free_tree[drm_buddy_block_order(block)];

+    struct rb_node **link = &root->rb_node;
+    struct rb_node *parent = NULL;
      struct drm_buddy_block *node;
-    struct list_head *head;
+    u64 offset;
+
+    offset = drm_buddy_block_offset(block);
  -    head = &mm->free_list[drm_buddy_block_order(block)];
-    if (list_empty(head)) {
-        list_add(&block->link, head);
-        return;
+    while (*link) {
+        parent = *link;
+        node = rb_entry(parent, struct drm_buddy_block, rb);
+
+        if (offset < drm_buddy_block_offset(node))
+            link = &parent->rb_left;
+        else
+            link = &parent->rb_right;
      }
  -    list_for_each_entry(node, head, link)

-        if (drm_buddy_block_offset(block) < drm_buddy_block_offset(node))

-            break;
+    rb_link_node(&block->rb, parent, link);
+    rb_insert_color(&block->rb, root);
+}
+
+static void rbtree_remove(struct drm_buddy *mm,
+              struct drm_buddy_block *block)
+{
+    struct rb_root *root;
  -    __list_add(&block->link, node->link.prev, &node->link);
+    root = &mm->free_tree[drm_buddy_block_order(block)];
+    rb_erase(&block->rb, root);
+
+    RB_CLEAR_NODE(&block->rb);
+}
+
+static inline struct drm_buddy_block *
+rbtree_last_entry(struct drm_buddy *mm, unsigned int order)
+{
+    struct rb_node *node = rb_last(&mm->free_tree[order]);
+
+    return node ? rb_entry(node, struct drm_buddy_block, rb) : NULL;
+}
+
+static bool rbtree_is_empty(struct drm_buddy *mm, unsigned int order)
+{
+    return RB_EMPTY_ROOT(&mm->free_tree[order]);
  }
    static void clear_reset(struct drm_buddy_block *block)

@@ -70,12 +102,13 @@ static void mark_cleared(struct drm_buddy_block *block)

      block->header |= DRM_BUDDY_HEADER_CLEAR;
  }
  -static void mark_allocated(struct drm_buddy_block *block)
+static void mark_allocated(struct drm_buddy *mm,
+               struct drm_buddy_block *block)
  {
      block->header &= ~DRM_BUDDY_HEADER_STATE;
      block->header |= DRM_BUDDY_ALLOCATED;
  -    list_del(&block->link);
+    rbtree_remove(mm, block);
  }
    static void mark_free(struct drm_buddy *mm,
@@ -84,15 +117,16 @@ static void mark_free(struct drm_buddy *mm,
      block->header &= ~DRM_BUDDY_HEADER_STATE;
      block->header |= DRM_BUDDY_FREE;
  -    list_insert_sorted(mm, block);
+    rbtree_insert(mm, block);
  }
  -static void mark_split(struct drm_buddy_block *block)
+static void mark_split(struct drm_buddy *mm,
+               struct drm_buddy_block *block)
  {
      block->header &= ~DRM_BUDDY_HEADER_STATE;
      block->header |= DRM_BUDDY_SPLIT;
  -    list_del(&block->link);
+    rbtree_remove(mm, block);
  }
    static inline bool overlaps(u64 s1, u64 e1, u64 s2, u64 e2)

@@ -148,7 +182,7 @@ static unsigned int __drm_buddy_free(struct drm_buddy *mm,

                  mark_cleared(parent);
          }
  -        list_del(&buddy->link);
+        rbtree_remove(mm, buddy);
          if (force_merge && drm_buddy_block_is_clear(buddy))
              mm->clear_avail -= drm_buddy_block_size(mm, buddy);
  @@ -179,12 +213,17 @@ static int __force_merge(struct drm_buddy *mm,
          return -EINVAL;
        for (i = min_order - 1; i >= 0; i--) {
-        struct drm_buddy_block *block, *prev;
+        struct drm_buddy_block *block, *prev_block, *first_block;
+

+        first_block = rb_entry(rb_first(&mm->free_tree[i]), struct drm_buddy_block, rb);   -        list_for_each_entry_safe_reverse(block, prev, &mm- >free_list[i], link) { +        for_each_rb_entry_reverse_safe(block, prev_block, &mm- >free_tree[i], rb) {

              struct drm_buddy_block *buddy;
              u64 block_start, block_end;
  +            if (RB_EMPTY_NODE(&block->rb))
+                break;

If we got the block from the rb tree, can it be empty here?

I saw a crash earlier without this check while running graphics workloads, but it is not happening now. I will

test with more workloads and remove it if everything looks fine.

+
              if (!block->parent)
                  continue;
  @@ -206,10 +245,14 @@ static int __force_merge(struct drm_buddy *mm,
               * block in the next iteration as we would free the
               * buddy block as part of the free function.
               */
-            if (prev == buddy)
-                prev = list_prev_entry(prev, link);
+            if (prev_block && prev_block == buddy) {
+                if (prev_block != first_block)
+                    prev_block = rb_entry(rb_prev(&prev_block->rb),
+                                  struct drm_buddy_block,
+                                  rb);
+            }
  -            list_del(&block->link);
+            rbtree_remove(mm, block);
              if (drm_buddy_block_is_clear(block))
                  mm->clear_avail -= drm_buddy_block_size(mm, block);

  @@ -258,14 +301,14 @@ int drm_buddy_init(struct drm_buddy *mm, u64 size, u64 chunk_size)

        BUG_ON(mm->max_order > DRM_BUDDY_MAX_ORDER);
  -    mm->free_list = kmalloc_array(mm->max_order + 1,
-                      sizeof(struct list_head),
+    mm->free_tree = kmalloc_array(mm->max_order + 1,
+                      sizeof(struct rb_root),
                        GFP_KERNEL);
-    if (!mm->free_list)
+    if (!mm->free_tree)
          return -ENOMEM;
        for (i = 0; i <= mm->max_order; ++i)
-        INIT_LIST_HEAD(&mm->free_list[i]);
+        mm->free_tree[i] = RB_ROOT;
        mm->n_roots = hweight64(size);

  @@ -273,7 +316,7 @@ int drm_buddy_init(struct drm_buddy *mm, u64 size, u64 chunk_size)

                    sizeof(struct drm_buddy_block *),
                    GFP_KERNEL);
      if (!mm->roots)
-        goto out_free_list;
+        goto out_free_tree;
        offset = 0;
      i = 0;

@@ -312,8 +355,8 @@ int drm_buddy_init(struct drm_buddy *mm, u64 size, u64 chunk_size)

      while (i--)
          drm_block_free(mm, mm->roots[i]);
      kfree(mm->roots);
-out_free_list:
-    kfree(mm->free_list);
+out_free_tree:
+    kfree(mm->free_tree);
      return -ENOMEM;
  }
  EXPORT_SYMBOL(drm_buddy_init);
@@ -323,7 +366,7 @@ EXPORT_SYMBOL(drm_buddy_init);
   *
   * @mm: DRM buddy manager to free
   *
- * Cleanup memory manager resources and the freelist
+ * Cleanup memory manager resources and the freetree
   */
  void drm_buddy_fini(struct drm_buddy *mm)
  {
@@ -350,7 +393,7 @@ void drm_buddy_fini(struct drm_buddy *mm)
      WARN_ON(mm->avail != mm->size);
        kfree(mm->roots);
-    kfree(mm->free_list);
+    kfree(mm->free_tree);
  }
  EXPORT_SYMBOL(drm_buddy_fini);
  @@ -383,7 +426,7 @@ static int split_block(struct drm_buddy *mm,
          clear_reset(block);
      }
  -    mark_split(block);
+    mark_split(mm, block);
        return 0;
  }

@@ -598,7 +641,7 @@ get_maxblock(struct drm_buddy *mm, unsigned int order,

      for (i = order; i <= mm->max_order; ++i) {
          struct drm_buddy_block *tmp_block;

  -        list_for_each_entry_reverse(tmp_block, &mm->free_list[i], link) {

+        for_each_rb_entry_reverse(tmp_block, &mm->free_tree[i], rb) {
              if (block_incompatible(tmp_block, flags))
                  continue;

  @@ -624,7 +667,7 @@ get_maxblock(struct drm_buddy *mm, unsigned int order,

  }
    static struct drm_buddy_block *
-alloc_from_freelist(struct drm_buddy *mm,
+alloc_from_freetree(struct drm_buddy *mm,
              unsigned int order,
              unsigned long flags)
  {
@@ -641,7 +684,7 @@ alloc_from_freelist(struct drm_buddy *mm,
          for (tmp = order; tmp <= mm->max_order; ++tmp) {
              struct drm_buddy_block *tmp_block;

  -            list_for_each_entry_reverse(tmp_block, &mm- >free_list[tmp], link) { +            for_each_rb_entry_reverse(tmp_block, &mm- >free_tree[tmp], rb) {

                  if (block_incompatible(tmp_block, flags))
                      continue;
  @@ -657,10 +700,8 @@ alloc_from_freelist(struct drm_buddy *mm,
      if (!block) {
          /* Fallback method */
          for (tmp = order; tmp <= mm->max_order; ++tmp) {
-            if (!list_empty(&mm->free_list[tmp])) {
-                block = list_last_entry(&mm->free_list[tmp],
-                            struct drm_buddy_block,
-                            link);
+            if (!rbtree_is_empty(mm, tmp)) {
+                block = rbtree_last_entry(mm, tmp);
                  if (block)
                      break;
              }
@@ -728,7 +769,7 @@ static int __alloc_range(struct drm_buddy *mm,
            if (contains(start, end, block_start, block_end)) {
              if (drm_buddy_block_is_free(block)) {
-                mark_allocated(block);
+                mark_allocated(mm, block);
                  total_allocated += drm_buddy_block_size(mm, block);
                  mm->avail -= drm_buddy_block_size(mm, block);
                  if (drm_buddy_block_is_clear(block))

@@ -806,7 +847,6 @@ static int __alloc_contig_try_harder(struct drm_buddy *mm,

  {
      u64 rhs_offset, lhs_offset, lhs_size, filled;
      struct drm_buddy_block *block;
-    struct list_head *list;
      LIST_HEAD(blocks_lhs);
      unsigned long pages;
      unsigned int order;

@@ -819,11 +859,10 @@ static int __alloc_contig_try_harder(struct drm_buddy *mm,

      if (order == 0)
          return -ENOSPC;
  -    list = &mm->free_list[order];
-    if (list_empty(list))
+    if (rbtree_is_empty(mm, order))
          return -ENOSPC;
  -    list_for_each_entry_reverse(block, list, link) {
+    for_each_rb_entry_reverse(block, &mm->free_tree[order], rb) {
          /* Allocate blocks traversing RHS */
          rhs_offset = drm_buddy_block_offset(block);
          err =  __drm_buddy_alloc_range(mm, rhs_offset, size,
@@ -933,7 +972,7 @@ int drm_buddy_block_trim(struct drm_buddy *mm,
      list_add(&block->tmp_link, &dfs);

      err =  __alloc_range(mm, &dfs, new_start, new_size, blocks, NULL);

      if (err) {
-        mark_allocated(block);
+        mark_allocated(mm, block);
          mm->avail -= drm_buddy_block_size(mm, block);
          if (drm_buddy_block_is_clear(block))
              mm->clear_avail -= drm_buddy_block_size(mm, block);
@@ -956,8 +995,8 @@ __drm_buddy_alloc_blocks(struct drm_buddy *mm,
          return  __drm_buddy_alloc_range_bias(mm, start, end,
                               order, flags);
      else
-        /* Allocate from freelist */
-        return alloc_from_freelist(mm, order, flags);
+        /* Allocate from freetree */
+        return alloc_from_freetree(mm, order, flags);
  }
    /**
@@ -974,8 +1013,8 @@ __drm_buddy_alloc_blocks(struct drm_buddy *mm,
   * alloc_range_bias() called on range limitations, which traverses
   * the tree and returns the desired block.
   *
- * alloc_from_freelist() called when *no* range restrictions
- * are enforced, which picks the block from the freelist.
+ * alloc_from_freetree() called when *no* range restrictions
+ * are enforced, which picks the block from the freetree.
   *
   * Returns:
   * 0 on success, error code on failure.
@@ -1077,7 +1116,7 @@ int drm_buddy_alloc_blocks(struct drm_buddy *mm,
              }
          } while (1);
  -        mark_allocated(block);
+        mark_allocated(mm, block);
          mm->avail -= drm_buddy_block_size(mm, block);
          if (drm_buddy_block_is_clear(block))
              mm->clear_avail -= drm_buddy_block_size(mm, block);

@@ -1161,7 +1200,7 @@ void drm_buddy_print(struct drm_buddy *mm, struct drm_printer *p)

          struct drm_buddy_block *block;
          u64 count = 0, free;
  -        list_for_each_entry(block, &mm->free_list[order], link) {
+        for_each_rb_entry(block, &mm->free_tree[order], rb) {
              BUG_ON(!drm_buddy_block_is_free(block));
              count++;
          }
diff --git a/include/drm/drm_buddy.h b/include/drm/drm_buddy.h
index 9689a7c5dd36..a64d108a33b7 100644
--- a/include/drm/drm_buddy.h
+++ b/include/drm/drm_buddy.h
@@ -10,6 +10,7 @@
  #include <linux/list.h>
  #include <linux/slab.h>
  #include <linux/sched.h>
+#include <linux/rbtree.h>
    #include <drm/drm_print.h>
  @@ -22,6 +23,41 @@
      start__ >= max__ || size__ > max__ - start__; \
  })
  +/*
+ * for_each_rb_entry() - iterate over an RB tree in order
+ * @pos:    the struct type * to use as a loop cursor
+ * @root:    pointer to struct rb_root to iterate
+ * @member:    name of the rb_node field within the struct
+ */
+#define for_each_rb_entry(pos, root, member) \
+    for (pos = rb_entry_safe(rb_first(root), typeof(*pos), member); \
+         pos; \

+         pos = rb_entry_safe(rb_next(&(pos)->member), typeof(*pos), member))

+
+/*

+ * for_each_rb_entry_reverse() - iterate over an RB tree in reverse order

+ * @pos:    the struct type * to use as a loop cursor
+ * @root:    pointer to struct rb_root to iterate
+ * @member:    name of the rb_node field within the struct
+ */
+#define for_each_rb_entry_reverse(pos, root, member) \
+    for (pos = rb_entry_safe(rb_last(root), typeof(*pos), member); \
+         pos; \

+         pos = rb_entry_safe(rb_prev(&(pos)->member), typeof(*pos), member))

+
+/**

+ * for_each_rb_entry_reverse_safe() - safely iterate over an RB tree in reverse order

+ * @pos:    the struct type * to use as a loop cursor.
+ * @n:        another struct type * to use as temporary storage.
+ * @root:    pointer to struct rb_root to iterate.
+ * @member:    name of the rb_node field within the struct.
+ */
+#define for_each_rb_entry_reverse_safe(pos, n, root, member) \

Would it make sense to give these a less generic name? Something like for_each_rb_free_block_* ?

Also should this be exported or rather kept within .c?

yes, I will change it.

+    for (pos = rb_entry_safe(rb_last(root), typeof(*pos), member), \

+         n = pos ? rb_entry_safe(rb_prev(&(pos)->member), typeof(*pos), member) : NULL; \

+         pos; \

+         pos = n, n = pos ? rb_entry_safe(rb_prev(&(pos)->member), typeof(*pos), member) : NULL)

+
  #define DRM_BUDDY_RANGE_ALLOCATION        BIT(0)
  #define DRM_BUDDY_TOPDOWN_ALLOCATION        BIT(1)
  #define DRM_BUDDY_CONTIGUOUS_ALLOCATION        BIT(2)
@@ -53,6 +89,7 @@ struct drm_buddy_block {
       * a list, if so desired. As soon as the block is freed with
       * drm_buddy_free* ownership is given back to the mm.
       */
+    struct rb_node rb;
      struct list_head link;

I think we can be slippery here and make this a union? They should be mutually exclusive AFAICT?

AFAIK, we need both, rb_node rb is for handling free blocks and the list_head link for adding the allocated blocks into

the driver's list.

At a given time I think it is either on the free rb or user block list, not both at the same time, given that a block can't be free and allocated? Just thinking if there is an easy way to trim the size a bit, given that we are adding an entire rb_node, and there can be many of these of these blocks around.

yes, you are right. I will add union for free rb and user block list.

      struct list_head tmp_link;

Otherwise it should be possible to get rid of this instead, and just re-use link? Could be done as separate patch, if this makes sense.

I think we cannot use link here since it is needed to add the allocated blocks to the driver's list and tmp_link is already used in

alloc_range() and alloc_range_bias().

Yeah, I don't this will work, but union might still be an option instead.

sure, we can make union for rb and list.

Regards,

Arun.

Regards,

Arun.

  };
@@ -68,7 +105,7 @@ struct drm_buddy_block {
   */
  struct drm_buddy {
      /* Maintain a free list for each order. */
-    struct list_head *free_list;
+    struct rb_root *free_tree;
        /*

       * Maintain explicit binary tree(s) to track the allocation of the