[Intel-gfx] [PATCH v2 1/3] drm/i915: Enable lockless lookup of request tracking via RCU

[Chris Wilson]

If we enable RCU for the requests (providing a grace period where we can
inspect a "dead" request before it is freed), we can allow callers to
carefully perform lockless lookup of an active request.

However, by enabling deferred freeing of requests, we can potentially
hog a lot of memory when dealing with tens of thousands of requests per
second - with a quick insertion of the a synchronize_rcu() inside our
shrinker callback, that issue disappears.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem.c          |  3 ++-
 drivers/gpu/drm/i915/i915_gem_request.c  |  2 +-
 drivers/gpu/drm/i915/i915_gem_request.h  | 24 +++++++++++++++++++++++-
 drivers/gpu/drm/i915/i915_gem_shrinker.c |  1 +
 4 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index c169574758d5..696ada3891ed 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4222,7 +4222,8 @@ i915_gem_load(struct drm_device *dev)
 	dev_priv->requests =
 		kmem_cache_create("i915_gem_request",
 				  sizeof(struct drm_i915_gem_request), 0,
-				  SLAB_HWCACHE_ALIGN,
+				  SLAB_HWCACHE_ALIGN |
+				  SLAB_DESTROY_BY_RCU,
 				  NULL);
 
 	INIT_LIST_HEAD(&dev_priv->context_list);
diff --git a/drivers/gpu/drm/i915/i915_gem_request.c b/drivers/gpu/drm/i915/i915_gem_request.c
index 4e087143b1d2..380a5963d957 100644
--- a/drivers/gpu/drm/i915/i915_gem_request.c
+++ b/drivers/gpu/drm/i915/i915_gem_request.c
@@ -335,7 +335,7 @@ static void __i915_gem_request_retire_active(struct drm_i915_gem_request *req)
 		 * and pass along the auxiliary information.
 		 */
 		INIT_LIST_HEAD(&active->link);
-		active->request = NULL;
+		rcu_assign_pointer(active->request, NULL);
 
 		active->retire(active, req);
 	}
diff --git a/drivers/gpu/drm/i915/i915_gem_request.h b/drivers/gpu/drm/i915/i915_gem_request.h
index 4eb15ed9205e..fae85b111a1e 100644
--- a/drivers/gpu/drm/i915/i915_gem_request.h
+++ b/drivers/gpu/drm/i915/i915_gem_request.h
@@ -139,6 +139,12 @@ i915_gem_request_get(struct drm_i915_gem_request *req)
 	return to_request(fence_get(&req->fence));
 }
 
+static inline struct drm_i915_gem_request *
+i915_gem_request_get_rcu(struct drm_i915_gem_request *req)
+{
+	return to_request(fence_get_rcu(&req->fence));
+}
+
 static inline void
 i915_gem_request_put(struct drm_i915_gem_request *req)
 {
@@ -243,7 +249,23 @@ i915_gem_request_mark_active(struct drm_i915_gem_request *request,
 			     struct i915_gem_active *active)
 {
 	list_move(&active->link, &request->active_list);
-	active->request = request;
+	rcu_assign_pointer(active->request, request);
+}
+
+static inline struct drm_i915_gem_request *
+i915_gem_active_get_request_rcu(struct i915_gem_active *active)
+{
+	do {
+		struct drm_i915_gem_request *request;
+
+		request = rcu_dereference(active->request);
+		if (request == NULL)
+			return NULL;
+
+		request = i915_gem_request_get_rcu(request);
+		if (request)
+			return request;
+	} while (1);
 }
 
 #endif /* I915_GEM_REQUEST_H */
diff --git a/drivers/gpu/drm/i915/i915_gem_shrinker.c b/drivers/gpu/drm/i915/i915_gem_shrinker.c
index c561ed2b8287..03a8bbb3e31e 100644
--- a/drivers/gpu/drm/i915/i915_gem_shrinker.c
+++ b/drivers/gpu/drm/i915/i915_gem_shrinker.c
@@ -142,6 +142,7 @@ i915_gem_shrink(struct drm_i915_private *dev_priv,
 	}
 
 	i915_gem_retire_requests(dev_priv->dev);
+	synchronize_rcu(); /* expedite the grace period to free the requests */
 
 	return count;
 }
-- 
2.6.4