backends/native: Use rtkit to get realtime priority

Instead of using sched_setscheduler directly (and relying that we
somehow got CAP_SYS_NICE), use rtkit to do this for us.

Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2284
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2060>

data/org.gnome.mutter.gschema.xml.in

@@ -122,9 +122,7 @@
                                         require a restart.
 
         • “rt-scheduler”              — makes mutter request a low priority
-                                        real-time scheduling. The executable
+                                        real-time scheduling. Requires a restart.
-                                        or user must have CAP_SYS_NICE.
-                                        Requires a restart.
 
         • “dma-buf-screen-sharing"    — enables DMA buffered screen sharing. This
                                         is already enabled by default when using

src/backends/native/meta-backend-native.c

@@ -38,7 +38,6 @@
 #include "backends/native/meta-backend-native-private.h"
 #include "backends/native/meta-input-thread.h"
 
-#include <sched.h>
 #include <stdlib.h>
 
 #include "backends/meta-cursor-tracker-private.h"
@@ -61,6 +60,7 @@
 #include "cogl/cogl.h"
 #include "core/meta-border.h"
 #include "meta/main.h"
+#include "meta-dbus-rtkit1.h"
 
 #ifdef HAVE_REMOTE_DESKTOP
 #include "backends/meta-screen-cast.h"
@@ -209,15 +209,36 @@ meta_backend_native_post_init (MetaBackend *backend)
   if (meta_settings_is_experimental_feature_enabled (settings,
                                                      META_EXPERIMENTAL_FEATURE_RT_SCHEDULER))
     {
-      int retval;
+      g_autoptr (MetaDbusRealtimeKit1) rtkit_proxy = NULL;
-      struct sched_param sp = {
+      g_autoptr (GError) error = NULL;
-        .sched_priority = sched_get_priority_min (SCHED_RR)
-      };
 
-      retval = sched_setscheduler (0, SCHED_RR | SCHED_RESET_ON_FORK, &sp);
+      rtkit_proxy =
+        meta_dbus_realtime_kit1_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
+                                                        G_DBUS_PROXY_FLAGS_DO_NOT_LOAD_PROPERTIES |
+                                                        G_DBUS_PROXY_FLAGS_DO_NOT_CONNECT_SIGNALS |
+                                                        G_DBUS_PROXY_FLAGS_DO_NOT_AUTO_START,
+                                                        "org.freedesktop.RealtimeKit1",
+                                                        "/org/freedesktop/RealtimeKit1",
+                                                        NULL,
+                                                        &error);
 
-      if (retval != 0)
+      if (rtkit_proxy)
-        g_warning ("Failed to set RT scheduler: %m");
+        {
+          uint32_t priority;
+
+          priority = sched_get_priority_min (SCHED_RR);
+          meta_dbus_realtime_kit1_call_make_thread_realtime_sync (rtkit_proxy,
+                                                                  gettid (),
+                                                                  priority,
+                                                                  NULL,
+                                                                  &error);
+        }
+
+      if (error)
+        {
+          g_dbus_error_strip_remote_error (error);
+          g_message ("Failed to set RT scheduler: %s", error->message);
+        }
     }
 
 #ifdef HAVE_REMOTE_DESKTOP

src/meson.build

@@ -891,6 +891,13 @@ if have_remote_desktop
   mutter_built_sources += dbus_screen_cast_built_sources
 endif
 
+dbus_rtkit_built_sources = gnome.gdbus_codegen('meta-dbus-rtkit1',
+    'org.freedesktop.RealtimeKit1.xml',
+    interface_prefix: 'org.freedesktop.',
+    namespace: 'MetaDbus',
+)
+mutter_built_sources += dbus_rtkit_built_sources
+
 wayland_protocol_server_headers = []
 wayland_protocol_client_headers = []
 wayland_protocol_sources = []

src/org.freedesktop.RealtimeKit1.xml

@@ -0,0 +1,42 @@
+<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
+"http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
+<node>
+        <interface name="org.freedesktop.RealtimeKit1">
+                <method name="MakeThreadRealtime">
+                        <arg name="thread" type="t" direction="in"/>
+                        <arg name="priority" type="u" direction="in"/>
+                </method>
+                <method name="MakeThreadRealtimeWithPID">
+                        <arg name="process" type="t" direction="in"/>
+                        <arg name="thread" type="t" direction="in"/>
+                        <arg name="priority" type="u" direction="in"/>
+                </method>
+                <method name="MakeThreadHighPriority">
+                        <arg name="thread" type="t" direction="in"/>
+                        <arg name="priority" type="i" direction="in"/>
+                </method>
+                <method name="MakeThreadHighPriorityWithPID">
+                        <arg name="process" type="t" direction="in"/>
+                        <arg name="thread" type="t" direction="in"/>
+                        <arg name="priority" type="i" direction="in"/>
+                </method>
+                <method name="ResetKnown"/>
+                <method name="ResetAll"/>
+                <method name="Exit"/>
+                <property name="RTTimeUSecMax" type="x" access="read"/>
+                <property name="MaxRealtimePriority" type="i" access="read"/>
+                <property name="MinNiceLevel" type="i" access="read"/>
+        </interface>
+        <interface name="org.freedesktop.DBus.Properties">
+                <method name="Get">
+                       <arg name="interface" direction="in" type="s"/>
+                       <arg name="property" direction="in" type="s"/>
+                       <arg name="value" direction="out" type="v"/>
+                </method>
+        </interface>
+        <interface name="org.freedesktop.DBus.Introspectable">
+                <method name="Introspect">
+                        <arg name="data" type="s" direction="out"/>
+                </method>
+        </interface>
+</node>