[PATCH 6/7] drm/syncobj: Add a fast path to drm_syncobj_array_wait_timeout

[Tvrtko Ursulin]

From: Tvrtko Ursulin <tvrtko.ursulin at igalia.com>

Running the Cyberpunk 2077 benchmark we can observe that waiting on DRM
sycobjs is relatively hot, but the 96% of the calls are for a single
object. (~4% for two points, and never more than three points. While
a more trivial workload like vkmark under Plasma is even more skewed
to single point waits.)

Therefore lets add a fast path to bypass the kcalloc/kfree and use a pre-
allocated stack array for those cases.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at igalia.com>
---
 drivers/gpu/drm/drm_syncobj.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
index b4563c696056..94932b89298f 100644
--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -1035,6 +1035,7 @@ static signed long drm_syncobj_array_wait_timeout(struct drm_syncobj **syncobjs,
 						  uint32_t *idx,
 						  ktime_t *deadline)
 {
+	struct syncobj_wait_entry stack_entries[4];
 	struct syncobj_wait_entry *entries;
 	uint32_t signaled_count, i;
 	struct dma_fence *fence;
@@ -1049,9 +1050,14 @@ static signed long drm_syncobj_array_wait_timeout(struct drm_syncobj **syncobjs,
 	    !access_ok(user_points, count * sizeof(*user_points)))
 		return -EFAULT;
 
-	entries = kcalloc(count, sizeof(*entries), GFP_KERNEL);
-	if (!entries)
-		return -ENOMEM;
+	if (count > ARRAY_SIZE(stack_entries)) {
+		entries = kcalloc(count, sizeof(*entries), GFP_KERNEL);
+		if (!entries)
+			return -ENOMEM;
+	} else {
+		memset(stack_entries, 0, sizeof(stack_entries));
+		entries = stack_entries;
+	}
 
 	/* Walk the list of sync objects and initialize entries.  We do
 	 * this up-front so that we can properly return -EINVAL if there is
@@ -1174,7 +1180,9 @@ static signed long drm_syncobj_array_wait_timeout(struct drm_syncobj **syncobjs,
 						  &entries[i].fence_cb);
 		dma_fence_put(entries[i].fence);
 	}
-	kfree(entries);
+
+	if (entries != stack_entries)
+		kfree(entries);
 
 	return timeout;
 }
-- 
2.47.1