core: disable realtime scheduling by default

And add a --realtime option for enabling it.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/config.h

@@ -152,6 +152,8 @@ typedef struct options {
 	bool no_x_selection;
 	/// Window type option override.
 	win_option_t wintype_option[NUM_WINTYPES];
+	/// Whether to set realtime scheduling policy for the compositor process.
+	bool use_realtime_scheduling;
 
 	// === VSync & software optimization ===
 	/// VSync method to use;

src/options.c

@@ -185,6 +185,8 @@ static const struct picom_option picom_options[] = {
                                                                              "you want to attach a debugger to picom"},
     {"no-ewmh-fullscreen"          , no_argument      , 803, NULL          , "Do not use EWMH to detect fullscreen windows. Reverts to checking if a "
                                                                              "window is fullscreen based only on its size and coordinates."},
+    {"realtime"                    , no_argument      , 804, NULL          , "Enable realtime scheduling. This might reduce latency, but might also cause "
+                                                                             "other issues. Disable this if you see the compositor being killed."},
 };
 // clang-format on
 
@@ -750,6 +752,7 @@ bool get_cfg(options_t *opt, int argc, char *const *argv, bool shadow_enable,
 			break;
 		P_CASEBOOL(802, debug_mode);
 		P_CASEBOOL(803, no_ewmh_fullscreen);
+		P_CASEBOOL(804, use_realtime_scheduling);
 		default: usage(argv[0], 1); break;
 #undef P_CASEBOOL
 		}

src/picom.c

@@ -1569,8 +1569,9 @@ static bool redirect_start(session_t *ps) {
 			    (enum vblank_scheduler_type)ps->o.debug_options.force_vblank_scheduler;
 		}
 		log_info("Using vblank scheduler: %s.", vblank_scheduler_str[scheduler_type]);
-		ps->vblank_scheduler = vblank_scheduler_new(
-		    ps->loop, &ps->c, session_get_target_window(ps), scheduler_type);
+		ps->vblank_scheduler =
+		    vblank_scheduler_new(ps->loop, &ps->c, session_get_target_window(ps),
+		                         scheduler_type, ps->o.use_realtime_scheduling);
 		if (!ps->vblank_scheduler) {
 			return false;
 		}
@@ -2740,7 +2741,10 @@ static void session_destroy(session_t *ps) {
  * @param ps current session
  */
 static void session_run(session_t *ps) {
-	set_rr_scheduling();
+	if (ps->o.use_realtime_scheduling) {
+		set_rr_scheduling();
+	}
+
 	// In benchmark mode, we want draw_timer handler to always be active
 	if (ps->o.benchmark) {
 		ev_timer_set(&ps->draw_timer, 0, 0);

src/vblank.c

@@ -47,6 +47,7 @@ struct vblank_scheduler {
 	xcb_window_t target_window;
 	enum vblank_scheduler_type type;
 	bool vblank_event_requested;
+	bool use_realtime_scheduling;
 };
 
 struct present_vblank_scheduler {
@@ -92,6 +93,7 @@ struct sgi_video_sync_vblank_scheduler {
 struct sgi_video_sync_thread_args {
 	struct sgi_video_sync_vblank_scheduler *self;
 	int start_status;
+	bool use_realtime_scheduling;
 	pthread_mutex_t start_mtx;
 	pthread_cond_t start_cnd;
 };
@@ -184,7 +186,11 @@ static void *sgi_video_sync_thread(void *data) {
 	}
 
 	log_init_tls();
-	set_rr_scheduling();
+
+	if (args->use_realtime_scheduling) {
+		set_rr_scheduling();
+	}
+
 	pthread_mutex_lock(&args->start_mtx);
 	args->start_status = 0;
 	pthread_cond_signal(&args->start_cnd);
@@ -258,6 +264,7 @@ static bool sgi_video_sync_scheduler_init(struct vblank_scheduler *base) {
 	auto args = (struct sgi_video_sync_thread_args){
 	    .self = self,
 	    .start_status = -1,
+	    .use_realtime_scheduling = base->use_realtime_scheduling,
 	};
 	bool succeeded = true;
 	pthread_mutex_init(&args.start_mtx, NULL);
@@ -543,8 +550,8 @@ void vblank_scheduler_free(struct vblank_scheduler *self) {
 }
 
 struct vblank_scheduler *
-vblank_scheduler_new(struct ev_loop *loop, struct x_connection *c,
-                     xcb_window_t target_window, enum vblank_scheduler_type type) {
+vblank_scheduler_new(struct ev_loop *loop, struct x_connection *c, xcb_window_t target_window,
+                     enum vblank_scheduler_type type, bool use_realtime_scheduling) {
 	size_t object_size = vblank_scheduler_ops[type].size;
 	auto init_fn = vblank_scheduler_ops[type].init;
 	if (!object_size || !init_fn) {
@@ -557,6 +564,7 @@ vblank_scheduler_new(struct ev_loop *loop, struct x_connection *c,
 	self->target_window = target_window;
 	self->c = c;
 	self->loop = loop;
+	self->use_realtime_scheduling = use_realtime_scheduling;
 	init_fn(self);
 	return self;
 }

src/vblank.h

@@ -40,8 +40,8 @@ typedef enum vblank_callback_action (*vblank_callback_t)(struct vblank_event *ev
 bool vblank_scheduler_schedule(struct vblank_scheduler *self, vblank_callback_t cb,
                                void *user_data);
 struct vblank_scheduler *
-vblank_scheduler_new(struct ev_loop *loop, struct x_connection *c,
-                     xcb_window_t target_window, enum vblank_scheduler_type type);
+vblank_scheduler_new(struct ev_loop *loop, struct x_connection *c, xcb_window_t target_window,
+                     enum vblank_scheduler_type type, bool use_realtime_scheduling);
 void vblank_scheduler_free(struct vblank_scheduler *);
 
 bool vblank_handle_x_events(struct vblank_scheduler *self);