drm/i915: Allow contexts to be unreferenced locklessly

If we move the actual cleanup of the context to a worker, we can allow
the final free to be called from any context and avoid undue latency in
the caller.

v2: Negotiate handling the delayed contexts free by flushing the
workqueue before calling i915_gem_context_fini() and performing the final
free of the kernel context directly
v3: Flush deferred frees before new context allocations

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170620110547.15947-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gvt/scheduler.c

@@ -620,7 +620,7 @@ err:
 
 void intel_vgpu_clean_gvt_context(struct intel_vgpu *vgpu)
 {
-	i915_gem_context_put_unlocked(vgpu->shadow_ctx);
+	i915_gem_context_put(vgpu->shadow_ctx);
 }
 
 int intel_vgpu_init_gvt_context(struct intel_vgpu *vgpu)

drivers/gpu/drm/i915/i915_drv.c

@@ -585,6 +585,8 @@ static const struct vga_switcheroo_client_ops i915_switcheroo_ops = {
 
 static void i915_gem_fini(struct drm_i915_private *dev_priv)
 {
+	flush_workqueue(dev_priv->wq);
+
 	mutex_lock(&dev_priv->drm.struct_mutex);
 	intel_uc_fini_hw(dev_priv);
 	i915_gem_cleanup_engines(dev_priv);

drivers/gpu/drm/i915/i915_drv.h

@@ -2315,6 +2315,8 @@ struct drm_i915_private {
 
 	struct {
 		struct list_head list;
+		struct llist_head free_list;
+		struct work_struct free_work;
 
 		/* The hw wants to have a stable context identifier for the
 		 * lifetime of the context (for OA, PASID, faults, etc).
@@ -3545,27 +3547,6 @@ i915_gem_context_lookup(struct drm_i915_file_private *file_priv, u32 id)
 	return ctx;
 }
 
-static inline struct i915_gem_context *
-i915_gem_context_get(struct i915_gem_context *ctx)
-{
-	kref_get(&ctx->ref);
-	return ctx;
-}
-
-static inline void i915_gem_context_put(struct i915_gem_context *ctx)
-{
-	lockdep_assert_held(&ctx->i915->drm.struct_mutex);
-	kref_put(&ctx->ref, i915_gem_context_free);
-}
-
-static inline void i915_gem_context_put_unlocked(struct i915_gem_context *ctx)
-{
-	struct mutex *lock = &ctx->i915->drm.struct_mutex;
-
-	if (kref_put_mutex(&ctx->ref, i915_gem_context_free, lock))
-		mutex_unlock(lock);
-}
-
 static inline struct intel_timeline *
 i915_gem_context_lookup_timeline(struct i915_gem_context *ctx,
 				 struct intel_engine_cs *engine)

drivers/gpu/drm/i915/i915_gem_context.c

@@ -158,13 +158,11 @@ static void vma_lut_free(struct i915_gem_context *ctx)
 	kvfree(lut->ht);
 }
 
-void i915_gem_context_free(struct kref *ctx_ref)
+static void i915_gem_context_free(struct i915_gem_context *ctx)
 {
-	struct i915_gem_context *ctx = container_of(ctx_ref, typeof(*ctx), ref);
 	int i;
 
 	lockdep_assert_held(&ctx->i915->drm.struct_mutex);
-	trace_i915_context_free(ctx);
 	GEM_BUG_ON(!i915_gem_context_is_closed(ctx));
 
 	vma_lut_free(ctx);
@@ -192,6 +190,37 @@ void i915_gem_context_free(struct kref *ctx_ref)
 	kfree(ctx);
 }
 
+static void contexts_free(struct drm_i915_private *i915)
+{
+	struct llist_node *freed = llist_del_all(&i915->contexts.free_list);
+	struct i915_gem_context *ctx;
+
+	lockdep_assert_held(&i915->drm.struct_mutex);
+
+	llist_for_each_entry(ctx, freed, free_link)
+		i915_gem_context_free(ctx);
+}
+
+static void contexts_free_worker(struct work_struct *work)
+{
+	struct drm_i915_private *i915 =
+		container_of(work, typeof(*i915), contexts.free_work);
+
+	mutex_lock(&i915->drm.struct_mutex);
+	contexts_free(i915);
+	mutex_unlock(&i915->drm.struct_mutex);
+}
+
+void i915_gem_context_release(struct kref *ref)
+{
+	struct i915_gem_context *ctx = container_of(ref, typeof(*ctx), ref);
+	struct drm_i915_private *i915 = ctx->i915;
+
+	trace_i915_context_free(ctx);
+	if (llist_add(&ctx->free_link, &i915->contexts.free_list))
+		queue_work(i915->wq, &i915->contexts.free_work);
+}
+
 static void context_close(struct i915_gem_context *ctx)
 {
 	i915_gem_context_set_closed(ctx);
@@ -428,6 +457,8 @@ int i915_gem_contexts_init(struct drm_i915_private *dev_priv)
 		return 0;
 
 	INIT_LIST_HEAD(&dev_priv->contexts.list);
+	INIT_WORK(&dev_priv->contexts.free_work, contexts_free_worker);
+	init_llist_head(&dev_priv->contexts.free_list);
 
 	if (intel_vgpu_active(dev_priv) &&
 	    HAS_LOGICAL_RING_CONTEXTS(dev_priv)) {
@@ -505,18 +536,20 @@ void i915_gem_contexts_lost(struct drm_i915_private *dev_priv)
 	}
 }
 
-void i915_gem_contexts_fini(struct drm_i915_private *dev_priv)
+void i915_gem_contexts_fini(struct drm_i915_private *i915)
 {
-	struct i915_gem_context *dctx = dev_priv->kernel_context;
-
-	lockdep_assert_held(&dev_priv->drm.struct_mutex);
+	struct i915_gem_context *ctx;
 
-	GEM_BUG_ON(!i915_gem_context_is_kernel(dctx));
+	lockdep_assert_held(&i915->drm.struct_mutex);
 
-	context_close(dctx);
-	dev_priv->kernel_context = NULL;
+	/* Keep the context so that we can free it immediately ourselves */
+	ctx = i915_gem_context_get(fetch_and_zero(&i915->kernel_context));
+	GEM_BUG_ON(!i915_gem_context_is_kernel(ctx));
+	context_close(ctx);
+	i915_gem_context_free(ctx);
 
-	ida_destroy(&dev_priv->contexts.hw_ida);
+	/* Must free all deferred contexts (via flush_workqueue) first */
+	ida_destroy(&i915->contexts.hw_ida);
 }
 
 static int context_idr_cleanup(int id, void *p, void *data)
@@ -957,6 +990,10 @@ int i915_gem_context_create_ioctl(struct drm_device *dev, void *data,
 	if (ret)
 		return ret;
 
+	/* Reap stale contexts */
+	i915_gem_retire_requests(dev_priv);
+	contexts_free(dev_priv);
+
 	ctx = i915_gem_create_context(dev_priv, file_priv);
 	mutex_unlock(&dev->struct_mutex);
 	if (IS_ERR(ctx))

drivers/gpu/drm/i915/i915_gem_context.h

@@ -86,6 +86,7 @@ struct i915_gem_context {
 
 	/** link: place with &drm_i915_private.context_list */
 	struct list_head link;
+	struct llist_node free_link;
 
 	/**
 	 * @ref: reference count
@@ -284,7 +285,7 @@ void i915_gem_context_close(struct drm_file *file);
 int i915_switch_context(struct drm_i915_gem_request *req);
 int i915_gem_switch_to_kernel_context(struct drm_i915_private *dev_priv);
 
-void i915_gem_context_free(struct kref *ctx_ref);
+void i915_gem_context_release(struct kref *ctx_ref);
 struct i915_gem_context *
 i915_gem_context_create_gvt(struct drm_device *dev);
 
@@ -299,4 +300,16 @@ int i915_gem_context_setparam_ioctl(struct drm_device *dev, void *data,
 int i915_gem_context_reset_stats_ioctl(struct drm_device *dev, void *data,
 				       struct drm_file *file);
 
+static inline struct i915_gem_context *
+i915_gem_context_get(struct i915_gem_context *ctx)
+{
+	kref_get(&ctx->ref);
+	return ctx;
+}
+
+static inline void i915_gem_context_put(struct i915_gem_context *ctx)
+{
+	kref_put(&ctx->ref, i915_gem_context_release);
+}
+
 #endif /* !__I915_GEM_CONTEXT_H__ */

drivers/gpu/drm/i915/i915_perf.c

@@ -2444,7 +2444,7 @@ static void i915_perf_destroy_locked(struct i915_perf_stream *stream)
 	list_del(&stream->link);
 
 	if (stream->ctx)
-		i915_gem_context_put_unlocked(stream->ctx);
+		i915_gem_context_put(stream->ctx);
 
 	kfree(stream);
 }
@@ -2633,7 +2633,7 @@ err_alloc:
 	kfree(stream);
 err_ctx:
 	if (specific_ctx)
-		i915_gem_context_put_unlocked(specific_ctx);
+		i915_gem_context_put(specific_ctx);
 err:
 	return ret;
 }

drivers/gpu/drm/i915/selftests/i915_vma.c

@@ -186,16 +186,20 @@ static int igt_vma_create(void *arg)
 				goto end;
 		}
 
-		list_for_each_entry_safe(ctx, cn, &contexts, link)
+		list_for_each_entry_safe(ctx, cn, &contexts, link) {
+			list_del_init(&ctx->link);
 			mock_context_close(ctx);
+		}
 	}
 
 end:
 	/* Final pass to lookup all created contexts */
 	err = create_vmas(i915, &objects, &contexts);
 out:
-	list_for_each_entry_safe(ctx, cn, &contexts, link)
+	list_for_each_entry_safe(ctx, cn, &contexts, link) {
+		list_del_init(&ctx->link);
 		mock_context_close(ctx);
+	}
 
 	list_for_each_entry_safe(obj, on, &objects, st_link)
 		i915_gem_object_put(obj);

drivers/gpu/drm/i915/selftests/mock_context.c

@@ -86,3 +86,12 @@ void mock_context_close(struct i915_gem_context *ctx)
 
 	i915_gem_context_put(ctx);
 }
+
+void mock_init_contexts(struct drm_i915_private *i915)
+{
+	INIT_LIST_HEAD(&i915->contexts.list);
+	ida_init(&i915->contexts.hw_ida);
+
+	INIT_WORK(&i915->contexts.free_work, contexts_free_worker);
+	init_llist_head(&i915->contexts.free_list);
+}

drivers/gpu/drm/i915/selftests/mock_context.h

@@ -25,6 +25,8 @@
 #ifndef __MOCK_CONTEXT_H
 #define __MOCK_CONTEXT_H
 
+void mock_init_contexts(struct drm_i915_private *i915);
+
 struct i915_gem_context *
 mock_context(struct drm_i915_private *i915,
 	     const char *name);

drivers/gpu/drm/i915/selftests/mock_gem_device.c

@@ -57,6 +57,7 @@ static void mock_device_release(struct drm_device *dev)
 
 	cancel_delayed_work_sync(&i915->gt.retire_work);
 	cancel_delayed_work_sync(&i915->gt.idle_work);
+	flush_workqueue(i915->wq);
 
 	mutex_lock(&i915->drm.struct_mutex);
 	for_each_engine(engine, i915, id)
@@ -160,7 +161,7 @@ struct drm_i915_private *mock_gem_device(void)
 	INIT_LIST_HEAD(&i915->mm.unbound_list);
 	INIT_LIST_HEAD(&i915->mm.bound_list);
 
-	ida_init(&i915->contexts.hw_ida);
+	mock_init_contexts(i915);
 
 	INIT_DELAYED_WORK(&i915->gt.retire_work, mock_retire_work_handler);
 	INIT_DELAYED_WORK(&i915->gt.idle_work, mock_idle_work_handler);