[PATCH v4] drm/xe/ufence: Signal ufence faster when possible

[Nirmoy Das]

When the backing fence is already signaled, the ufence can be
immediately signaled without queuing in the ordered work queue.
This should also reduce load from the xe ordered_wq and won't
block signaling a ufence which doesn't require any serialization.

v2: fix system_wq typo
v3: signal immediately instead of queuing in system_wq (Matt B)
v4: revert back to v2 of using workqueue because of locking issue
    and remote viewing a different mm struct.
    Use Xe's unordered_wq which should be less congested than global
    one.

Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/1630
Cc: Matthew Auld <matthew.auld at intel.com>
Cc: Matthew Brost <matthew.brost at intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das at intel.com>
---
 drivers/gpu/drm/xe/xe_sync.c | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_sync.c b/drivers/gpu/drm/xe/xe_sync.c
index a90480c6aecf..7a1558c7ce09 100644
--- a/drivers/gpu/drm/xe/xe_sync.c
+++ b/drivers/gpu/drm/xe/xe_sync.c
@@ -92,18 +92,27 @@ static void user_fence_worker(struct work_struct *w)
 	user_fence_put(ufence);
 }
 
-static void kick_ufence(struct xe_user_fence *ufence, struct dma_fence *fence)
+static void kick_ufence_ordered(struct xe_user_fence *ufence,
+				struct dma_fence *fence)
 {
 	INIT_WORK(&ufence->worker, user_fence_worker);
 	queue_work(ufence->xe->ordered_wq, &ufence->worker);
 	dma_fence_put(fence);
 }
 
+static void kick_ufence_unordered(struct xe_user_fence *ufence,
+				  struct dma_fence *fence)
+{
+	INIT_WORK(&ufence->worker, user_fence_worker);
+	queue_work(ufence->xe->unordered_wq, &ufence->worker);
+	dma_fence_put(fence);
+}
+
 static void user_fence_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
 {
 	struct xe_user_fence *ufence = container_of(cb, struct xe_user_fence, cb);
 
-	kick_ufence(ufence, fence);
+	kick_ufence_ordered(ufence, fence);
 }
 
 int xe_sync_entry_parse(struct xe_device *xe, struct xe_file *xef,
@@ -239,7 +248,16 @@ void xe_sync_entry_signal(struct xe_sync_entry *sync, struct dma_fence *fence)
 		err = dma_fence_add_callback(fence, &sync->ufence->cb,
 					     user_fence_cb);
 		if (err == -ENOENT) {
-			kick_ufence(sync->ufence, fence);
+			/*
+			 * use unordered_wq to schedule it faster and to keep
+			 * the ordered_wq less loaded as serialization is not
+			 * needed for when the fence is already signaled.
+			 *
+			 * This needs to be done with a wq here to avoid locking
+			 * issue when a ufence addr is backed by a bo and also
+			 * tsk->mm needs to null to call kthread_use_mm().
+			 */
+			kick_ufence_unordered(sync->ufence, fence);
 		} else if (err) {
 			XE_WARN_ON("failed to add user fence");
 			user_fence_put(sync->ufence);
-- 
2.46.0