[PATCH v2 3/3] gpu/buddy: Track per-order used blocks with a scoreboard

[Francois Dugast <francois.dugast@intel.com>]

Extend the scoreboard approach from the previous commit to used blocks,
so drm_buddy_print() can report per-order allocation pressure in O(1).

Unlike free blocks, an allocated block can leave the allocated state
through mark_free() (normal free and gpu_buddy_block_trim()) or be
consumed directly by gpu_block_free() during coalescing. Both sites are
guarded by gpu_buddy_block_is_allocated() and paired with the increment
in mark_allocated().

v2:
- Update after fix for use-after-free in split_block() call sites
- Change goto label to out_free_used_scoreboard for clarity
- Make drm_buddy_print() and gpu_buddy_print() symmetric for used and
  free

Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Assisted-by: GitHub Copilot:claude-sonnet-4.6
---
 drivers/gpu/buddy.c         | 39 +++++++++++++++++++++++++++----------
 drivers/gpu/drm/drm_buddy.c | 18 +++++++++++------
 include/linux/gpu_buddy.h   |  8 ++++++++
 3 files changed, 49 insertions(+), 16 deletions(-)

diff --git a/drivers/gpu/buddy.c b/drivers/gpu/buddy.c
index 01247e3201fb..daff39f50c40 100644
--- a/drivers/gpu/buddy.c
+++ b/drivers/gpu/buddy.c
@@ -194,6 +194,7 @@ static void mark_allocated(struct gpu_buddy *mm,
 	block->header |= GPU_BUDDY_ALLOCATED;
 
 	mm->free_scoreboard[gpu_buddy_block_order(block)]--;
+	mm->used_scoreboard[gpu_buddy_block_order(block)]++;
 
 	rbtree_remove(mm, block);
 }
@@ -203,6 +204,9 @@ static void mark_free(struct gpu_buddy *mm,
 {
 	enum gpu_buddy_free_tree tree;
 
+	if (gpu_buddy_block_is_allocated(block))
+		mm->used_scoreboard[gpu_buddy_block_order(block)]--;
+
 	block->header &= ~GPU_BUDDY_HEADER_STATE;
 	block->header |= GPU_BUDDY_FREE;
 
@@ -281,6 +285,9 @@ static unsigned int __gpu_buddy_free(struct gpu_buddy *mm,
 		if (force_merge && gpu_buddy_block_is_clear(buddy))
 			mm->clear_avail -= gpu_buddy_block_size(mm, buddy);
 
+		if (gpu_buddy_block_is_allocated(block))
+			mm->used_scoreboard[gpu_buddy_block_order(block)]--;
+
 		gpu_block_free(mm, block);
 		gpu_block_free(mm, buddy);
 
@@ -398,11 +405,17 @@ int gpu_buddy_init(struct gpu_buddy *mm, u64 size, u64 chunk_size)
 	if (!mm->free_scoreboard)
 		return -ENOMEM;
 
+	mm->used_scoreboard = kcalloc(mm->max_order + 1,
+				      sizeof(*mm->used_scoreboard),
+				      GFP_KERNEL);
+	if (!mm->used_scoreboard)
+		goto out_free_free_scoreboard;
+
 	mm->free_trees = kmalloc_array(GPU_BUDDY_MAX_FREE_TREES,
 				       sizeof(*mm->free_trees),
 				       GFP_KERNEL);
 	if (!mm->free_trees)
-		goto out_free_scoreboard;
+		goto out_free_used_scoreboard;
 
 	for_each_free_tree(i) {
 		mm->free_trees[i] = kmalloc_array(mm->max_order + 1,
@@ -464,7 +477,9 @@ int gpu_buddy_init(struct gpu_buddy *mm, u64 size, u64 chunk_size)
 	while (i--)
 		kfree(mm->free_trees[i]);
 	kfree(mm->free_trees);
-out_free_scoreboard:
+out_free_used_scoreboard:
+	kfree(mm->used_scoreboard);
+out_free_free_scoreboard:
 	kfree(mm->free_scoreboard);
 	return -ENOMEM;
 }
@@ -505,6 +520,7 @@ void gpu_buddy_fini(struct gpu_buddy *mm)
 	kfree(mm->free_trees);
 	kfree(mm->roots);
 	kfree(mm->free_scoreboard);
+	kfree(mm->used_scoreboard);
 }
 EXPORT_SYMBOL(gpu_buddy_fini);
 
@@ -1519,15 +1535,18 @@ void gpu_buddy_print(struct gpu_buddy *mm)
 		mm->chunk_size >> 10, mm->size >> 20, mm->avail >> 20, mm->clear_avail >> 20);
 
 	for (order = mm->max_order; order >= 0; order--) {
-		u64 count = mm->free_scoreboard[order];
-		u64 free = count * (mm->chunk_size << order);
-
-		if (free < SZ_1M)
-			pr_info("order-%2d free: %8llu KiB, blocks: %llu\n",
-				order, free >> 10, count);
+		u64 free_count = mm->free_scoreboard[order];
+		u64 used_count = mm->used_scoreboard[order];
+		u64 block_size = mm->chunk_size << order;
+		u64 free = free_count * block_size;
+		u64 used = used_count * block_size;
+
+		if (block_size < SZ_1M)
+			pr_info("order-%2d free: %8llu KiB, used: %8llu KiB, free_blocks: %llu, used_blocks: %llu\n",
+				order, free >> 10, used >> 10, free_count, used_count);
 		else
-			pr_info("order-%2d free: %8llu MiB, blocks: %llu\n",
-				order, free >> 20, count);
+			pr_info("order-%2d free: %8llu MiB, used: %8llu MiB, free_blocks: %llu, used_blocks: %llu\n",
+				order, free >> 20, used >> 20, free_count, used_count);
 	}
 }
 EXPORT_SYMBOL(gpu_buddy_print);
diff --git a/drivers/gpu/drm/drm_buddy.c b/drivers/gpu/drm/drm_buddy.c
index eef995e08a37..1536e59c6fe7 100644
--- a/drivers/gpu/drm/drm_buddy.c
+++ b/drivers/gpu/drm/drm_buddy.c
@@ -47,17 +47,23 @@ void drm_buddy_print(struct gpu_buddy *mm, struct drm_printer *p)
 		   mm->chunk_size >> 10, mm->size >> 20, mm->avail >> 20, mm->clear_avail >> 20);
 
 	for (order = mm->max_order; order >= 0; order--) {
-		u64 count = mm->free_scoreboard[order];
-		u64 free = count * (mm->chunk_size << order);
+		u64 free_count = mm->free_scoreboard[order];
+		u64 used_count = mm->used_scoreboard[order];
+		u64 block_size = mm->chunk_size << order;
+		u64 free = free_count * block_size;
+		u64 used = used_count * block_size;
 
 		drm_printf(p, "order-%2d ", order);
 
-		if (free < SZ_1M)
-			drm_printf(p, "free: %8llu KiB", free >> 10);
+		if (block_size < SZ_1M)
+			drm_printf(p, "free: %8llu KiB, used: %8llu KiB",
+				   free >> 10, used >> 10);
 		else
-			drm_printf(p, "free: %8llu MiB", free >> 20);
+			drm_printf(p, "free: %8llu MiB, used: %8llu MiB",
+				   free >> 20, used >> 20);
 
-		drm_printf(p, ", blocks: %llu\n", count);
+		drm_printf(p, ", free_blocks: %llu, used_blocks: %llu\n",
+			   free_count, used_count);
 	}
 }
 EXPORT_SYMBOL(drm_buddy_print);
diff --git a/include/linux/gpu_buddy.h b/include/linux/gpu_buddy.h
index a28f7d7637ca..e037714563d8 100644
--- a/include/linux/gpu_buddy.h
+++ b/include/linux/gpu_buddy.h
@@ -180,6 +180,14 @@ struct gpu_buddy {
 	 * called on a free block.
 	 */
 	u64 *free_scoreboard;
+	/*
+	 * Per-order used block scoreboard: used_scoreboard[order] holds the
+	 * number of blocks of that order currently in the allocated state.
+	 * Incremented in mark_allocated(), decremented in mark_free() (guarded
+	 * by gpu_buddy_block_is_allocated()) and in __gpu_buddy_free() when an
+	 * allocated block is consumed directly during buddy coalescing.
+	 */
+	u64 *used_scoreboard;
 /* public: */
 	unsigned int n_roots;
 	unsigned int max_order;
-- 
2.43.0