backends/native: Add rt-scheduler experimental key to set RT scheduling

This is similar to a change in kwin:
https://blog.martin-graesslin.com/blog/2017/09/kwinwayland-goes-real-time/

If the experimental features key has "rt-scheduler", make it claim the lowest
of RT scheduler priorities, this will be both educated to other RT processes
and improves responsiveness wrt all other processes.

This can only work if mutter/gnome-shell process receives CAP_SYS_NICE
somehow, e.g.: "setcap CAP_SYS_NICE=+ep `which gnome-shell`"

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/921
https://gitlab.gnome.org/GNOME/mutter/merge_requests/460

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

@@ -120,6 +120,10 @@
                                         framebuffers instead of window content,
                                         to manage HiDPI monitors. Does not
                                         require a restart.
+        • “rt-scheduler”              — makes mutter request a low priority
+                                        real-time scheduling. The executable
+                                        or user must have CAP_SYS_NICE.
+                                        Requires a restart.
       </description>
     </key>
 

src/backends/meta-settings-private.h

@@ -33,6 +33,7 @@ typedef enum _MetaExperimentalFeature
   META_EXPERIMENTAL_FEATURE_NONE = 0,
   META_EXPERIMENTAL_FEATURE_SCALE_MONITOR_FRAMEBUFFER = (1 << 0),
   META_EXPERIMENTAL_FEATURE_KMS_MODIFIERS  = (1 << 1),
+  META_EXPERIMENTAL_FEATURE_RT_SCHEDULER = (1 << 2),
 } MetaExperimentalFeature;
 
 #define META_TYPE_SETTINGS (meta_settings_get_type ())

src/backends/meta-settings.c

@@ -264,6 +264,8 @@ experimental_features_handler (GVariant *features_variant,
         features |= META_EXPERIMENTAL_FEATURE_SCALE_MONITOR_FRAMEBUFFER;
       else if (g_str_equal (feature, "kms-modifiers"))
         features |= META_EXPERIMENTAL_FEATURE_KMS_MODIFIERS;
+      else if (g_str_equal (feature, "rt-scheduler"))
+        features |= META_EXPERIMENTAL_FEATURE_RT_SCHEDULER;
       else
         g_info ("Unknown experimental feature '%s'\n", feature);
     }

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

@@ -37,6 +37,7 @@
 #include "backends/native/meta-backend-native.h"
 #include "backends/native/meta-backend-native-private.h"
 
+#include <sched.h>
 #include <stdlib.h>
 
 #include "backends/meta-cursor-tracker-private.h"
@@ -44,6 +45,7 @@
 #include "backends/meta-logical-monitor.h"
 #include "backends/meta-monitor-manager-private.h"
 #include "backends/meta-pointer-constraint.h"
+#include "backends/meta-settings-private.h"
 #include "backends/meta-stage-private.h"
 #include "backends/native/meta-barrier-native.h"
 #include "backends/native/meta-clutter-backend-native.h"
@@ -331,6 +333,7 @@ static void
 meta_backend_native_post_init (MetaBackend *backend)
 {
   ClutterDeviceManager *manager = clutter_device_manager_get_default ();
+  MetaSettings *settings = meta_backend_get_settings (backend);
 
   META_BACKEND_CLASS (meta_backend_native_parent_class)->post_init (backend);
 
@@ -338,6 +341,20 @@ meta_backend_native_post_init (MetaBackend *backend)
                                                 NULL, NULL);
   clutter_evdev_set_relative_motion_filter (manager, relative_motion_filter,
                                             meta_backend_get_monitor_manager (backend));
+
+  if (meta_settings_is_experimental_feature_enabled (settings,
+                                                     META_EXPERIMENTAL_FEATURE_RT_SCHEDULER))
+    {
+      int retval;
+      struct sched_param sp = {
+        .sched_priority = sched_get_priority_min (SCHED_RR)
+      };
+
+      retval = sched_setscheduler (0, SCHED_RR | SCHED_RESET_ON_FORK, &sp);
+
+      if (retval != 0)
+        g_warning ("Failed to set RT scheduler: %m");
+    }
 }
 
 static MetaMonitorManager *