sway: request SCHED_RR via RTKit

This tries to connect to DBUS to request RTTimeUSecMax from RTKit and
than changes the scheduling policy of sway to SCHED_RR. I tested the
impact using hackbench, with this patch I don't see any cursor
stuttering while running the following hackbench command:

  $ hackbench -f 100 -l 100000 -s 100000

This also fixes the "your compositor is too slow"-warnings from
libinput.

The rtkit feature is currently auto, so if one of the SD-Bus libraries
is installed we will select it. However we will fail graciously with
only a debug note if we can't reach RTKit and won't touch RLIMIT_RTTIME
in this case. Users who don't want to have DBUS within sway itself can
disable the feature.

include/sway/server.h

@@ -173,4 +173,6 @@ void handle_pointer_constraint(struct wl_listener *listener, void *data);
 void xdg_activation_v1_handle_request_activate(struct wl_listener *listener,
 	void *data);
 
+void gain_realtime(void);
+
 #endif

meson.build

@@ -106,6 +106,12 @@ if get_option('tray').enabled() and not tray_deps_found
 endif
 have_tray = (not get_option('tray').disabled()) and tray_deps_found
 
+rt_deps_found = sdbus.found()
+if get_option('rtkit').enabled() and not rt_deps_found
+	error('Building with -Drtkit=enabled, but sd-bus has not been not found')
+endif
+have_rtkit = (not get_option('rtkit').disabled()) and rt_deps_found
+
 conf_data = configuration_data()
 
 conf_data.set10('HAVE_XWAYLAND', have_xwayland)
@@ -114,6 +120,7 @@ conf_data.set10('HAVE_LIBSYSTEMD', sdbus.found() and sdbus.name() == 'libsystemd
 conf_data.set10('HAVE_LIBELOGIND', sdbus.found() and sdbus.name() == 'libelogind')
 conf_data.set10('HAVE_BASU', sdbus.found() and sdbus.name() == 'basu')
 conf_data.set10('HAVE_TRAY', have_tray)
+conf_data.set10('HAVE_RTKIT', have_rtkit)
 
 scdoc = dependency('scdoc', version: '>=1.9.2', native: true, required: get_option('man-pages'))
 if scdoc.found()
@@ -319,6 +326,7 @@ summary({
 	'xwayland': have_xwayland,
 	'gdk-pixbuf': gdk_pixbuf.found(),
 	'tray': have_tray,
+	'rtkit': have_rtkit,
 	'man-pages': scdoc.found(),
 }, bool_yn: true)
 

meson_options.txt

@@ -6,6 +6,7 @@ option('swaybar', type: 'boolean', value: true, description: 'Enable support for
 option('swaynag', type: 'boolean', value: true, description: 'Enable support for swaynag')
 option('xwayland', type: 'feature', value: 'auto', description: 'Enable support for X11 applications')
 option('tray', type: 'feature', value: 'auto', description: 'Enable support for swaybar tray')
+option('rtkit', type: 'feature', value: 'auto', description: 'Enable support to request realtime policy via RTKit')
 option('gdk-pixbuf', type: 'feature', value: 'auto', description: 'Enable support for more image formats in swaybg')
 option('man-pages', type: 'feature', value: 'auto', description: 'Generate and install man pages')
 option('sd-bus-provider', type: 'combo', choices: ['auto', 'libsystemd', 'libelogind', 'basu'], value: 'auto', description: 'Provider of the sd-bus library')

sway/main.c

@@ -413,6 +413,8 @@ int main(int argc, char **argv) {
 		goto shutdown;
 	}
 
+	gain_realtime();
+
 	if (!server_start(&server)) {
 		sway_terminate(EXIT_FAILURE);
 		goto shutdown;

sway/meson.build

@@ -7,6 +7,7 @@ sway_sources = files(
 	'ipc-server.c',
 	'lock.c',
 	'main.c',
+	'rtkit.c',
 	'server.c',
 	'swaynag.c',
 	'xdg_activation_v1.c',
@@ -229,6 +230,10 @@ if have_xwayland
 	sway_deps += xcb
 endif
 
+if have_rtkit
+	sway_deps += sdbus
+endif
+
 executable(
 	'sway',
 	sway_sources,

sway/rtkit.c

@@ -0,0 +1,71 @@
+#include <sys/resource.h>
+#include <sched.h>
+#include <unistd.h>
+#include "sway/server.h"
+#include "log.h"
+#if HAVE_LIBSYSTEMD
+#include <systemd/sd-bus.h>
+#elif HAVE_LIBELOGIND
+#include <elogind/sd-bus.h>
+#elif HAVE_BASU
+#include <basu/sd-bus.h>
+#endif
+
+#if HAVE_RTKIT
+
+void gain_realtime(void) {
+	sd_bus_error error = SD_BUS_ERROR_NULL;
+	sd_bus_message *m = NULL;
+	sd_bus *bus = NULL;
+	uint32_t prio = sched_get_priority_min(SCHED_RR);
+	int ret;
+	struct rlimit rl;
+	int max_usec;
+
+	ret = sd_bus_open_system(&bus);
+	if (ret < 0) {
+		sway_log(SWAY_DEBUG, "Failed to connect to system bus");
+		goto finish;
+	}
+
+	ret = sd_bus_get_property_trivial(bus, "org.freedesktop.RealtimeKit1",
+								"/org/freedesktop/RealtimeKit1", "org.freedesktop.RealtimeKit1",
+								"RTTimeUSecMax", &error, 'x', &max_usec);
+	if (ret < 0) {
+		sway_log(SWAY_DEBUG, "Couldn't query RTTimeUSecMax");
+		goto finish;
+	}
+
+	rl.rlim_cur = max_usec;
+	rl.rlim_max = max_usec;
+
+	if (setrlimit(RLIMIT_RTTIME, &rl) < 0) {
+		sway_log(SWAY_INFO, "Failed to setrlimit, no RT via RTKit possible");
+		goto finish;
+	}
+
+	ret = sd_bus_call_method(bus, "org.freedesktop.RealtimeKit1", "/org/freedesktop/RealtimeKit1",
+						"org.freedesktop.RealtimeKit1", "MakeThreadRealtime", &error, &m, "tu",
+						(uint64_t)getpid(), prio);
+
+	if (ret < 0) {
+		sway_log(SWAY_ERROR, "Failed to setup SCHED_RR: %s", error.message);
+		goto finish;
+	}
+	sway_log(SWAY_INFO, "Successfully setup SCHED_RR");
+
+finish:
+	sd_bus_error_free(&error);
+	sd_bus_message_unref(m);
+	sd_bus_unref(bus);
+	return;
+}
+
+#else
+
+void gain_realtime(void)
+{
+	return;
+}
+
+#endif