drm/ttm: optimize the pool shrinker a bit v2

Switch back to using a spinlock again by moving the IOMMU unmap outside
of the locked region.

This avoids contention especially while freeing pages.

v2: Add a comment explaining why we need sync_shrinkers().
Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210820120528.81114-3-christian.koenig@amd.com

drivers/gpu/drm/ttm/ttm_pool.c

@@ -70,7 +70,7 @@ static struct ttm_pool_type global_uncached[MAX_ORDER];
 static struct ttm_pool_type global_dma32_write_combined[MAX_ORDER];
 static struct ttm_pool_type global_dma32_uncached[MAX_ORDER];
 
-static struct mutex shrinker_lock;
+static spinlock_t shrinker_lock;
 static struct list_head shrinker_list;
 static struct shrinker mm_shrinker;
 
@@ -263,9 +263,9 @@ static void ttm_pool_type_init(struct ttm_pool_type *pt, struct ttm_pool *pool,
 	spin_lock_init(&pt->lock);
 	INIT_LIST_HEAD(&pt->pages);
 
-	mutex_lock(&shrinker_lock);
+	spin_lock(&shrinker_lock);
 	list_add_tail(&pt->shrinker_list, &shrinker_list);
-	mutex_unlock(&shrinker_lock);
+	spin_unlock(&shrinker_lock);
 }
 
 /* Remove a pool_type from the global shrinker list and free all pages */
@@ -273,9 +273,9 @@ static void ttm_pool_type_fini(struct ttm_pool_type *pt)
 {
 	struct page *p;
 
-	mutex_lock(&shrinker_lock);
+	spin_lock(&shrinker_lock);
 	list_del(&pt->shrinker_list);
-	mutex_unlock(&shrinker_lock);
+	spin_unlock(&shrinker_lock);
 
 	while ((p = ttm_pool_type_take(pt)))
 		ttm_pool_free_page(pt->pool, pt->caching, pt->order, p);
@@ -313,24 +313,23 @@ static struct ttm_pool_type *ttm_pool_select_type(struct ttm_pool *pool,
 static unsigned int ttm_pool_shrink(void)
 {
 	struct ttm_pool_type *pt;
-	unsigned int num_freed;
+	unsigned int num_pages;
 	struct page *p;
 
-	mutex_lock(&shrinker_lock);
+	spin_lock(&shrinker_lock);
 	pt = list_first_entry(&shrinker_list, typeof(*pt), shrinker_list);
+	list_move_tail(&pt->shrinker_list, &shrinker_list);
+	spin_unlock(&shrinker_lock);
 
 	p = ttm_pool_type_take(pt);
 	if (p) {
 		ttm_pool_free_page(pt->pool, pt->caching, pt->order, p);
-		num_freed = 1 << pt->order;
+		num_pages = 1 << pt->order;
 	} else {
-		num_freed = 0;
+		num_pages = 0;
 	}
 
-	list_move_tail(&pt->shrinker_list, &shrinker_list);
-	mutex_unlock(&shrinker_lock);
-
-	return num_freed;
+	return num_pages;
 }
 
 /* Return the allocation order based for a page */
@@ -530,6 +529,11 @@ void ttm_pool_fini(struct ttm_pool *pool)
 			for (j = 0; j < MAX_ORDER; ++j)
 				ttm_pool_type_fini(&pool->caching[i].orders[j]);
 	}
+
+	/* We removed the pool types from the LRU, but we need to also make sure
+	 * that no shrinker is concurrently freeing pages from the pool.
+	 */
+	synchronize_shrinkers();
 }
 
 /* As long as pages are available make sure to release at least one */
@@ -604,7 +608,7 @@ static int ttm_pool_debugfs_globals_show(struct seq_file *m, void *data)
 {
 	ttm_pool_debugfs_header(m);
 
-	mutex_lock(&shrinker_lock);
+	spin_lock(&shrinker_lock);
 	seq_puts(m, "wc\t:");
 	ttm_pool_debugfs_orders(global_write_combined, m);
 	seq_puts(m, "uc\t:");
@@ -613,7 +617,7 @@ static int ttm_pool_debugfs_globals_show(struct seq_file *m, void *data)
 	ttm_pool_debugfs_orders(global_dma32_write_combined, m);
 	seq_puts(m, "uc 32\t:");
 	ttm_pool_debugfs_orders(global_dma32_uncached, m);
-	mutex_unlock(&shrinker_lock);
+	spin_unlock(&shrinker_lock);
 
 	ttm_pool_debugfs_footer(m);
 
@@ -640,7 +644,7 @@ int ttm_pool_debugfs(struct ttm_pool *pool, struct seq_file *m)
 
 	ttm_pool_debugfs_header(m);
 
-	mutex_lock(&shrinker_lock);
+	spin_lock(&shrinker_lock);
 	for (i = 0; i < TTM_NUM_CACHING_TYPES; ++i) {
 		seq_puts(m, "DMA ");
 		switch (i) {
@@ -656,7 +660,7 @@ int ttm_pool_debugfs(struct ttm_pool *pool, struct seq_file *m)
 		}
 		ttm_pool_debugfs_orders(pool->caching[i].orders, m);
 	}
-	mutex_unlock(&shrinker_lock);
+	spin_unlock(&shrinker_lock);
 
 	ttm_pool_debugfs_footer(m);
 	return 0;
@@ -693,7 +697,7 @@ int ttm_pool_mgr_init(unsigned long num_pages)
 	if (!page_pool_size)
 		page_pool_size = num_pages;
 
-	mutex_init(&shrinker_lock);
+	spin_lock_init(&shrinker_lock);
 	INIT_LIST_HEAD(&shrinker_list);
 
 	for (i = 0; i < MAX_ORDER; ++i) {