[mutter/wip/carlosg/rt-scheduler] backends/native: Add rt-scheduler experimental key to set RT scheduling

[Carlos Garnacho <carlosg src gnome org>]

commit 7bee7c607b24eefa75996b6b459bc88ed8e74528
Author: Carlos Garnacho <carlosg gnome org>
Date:   Wed Feb 27 19:02:25 2019 +0100

    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      |  4 ++++
 src/backends/meta-settings-private.h      |  1 +
 src/backends/meta-settings.c              |  2 ++
 src/backends/native/meta-backend-native.c | 17 +++++++++++++++++
 4 files changed, 24 insertions(+)
---
diff --git a/data/org.gnome.mutter.gschema.xml.in b/data/org.gnome.mutter.gschema.xml.in
index 6cbd9c1b5..7bea9d66a 100644
--- a/data/org.gnome.mutter.gschema.xml.in
+++ b/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>
 
diff --git a/src/backends/meta-settings-private.h b/src/backends/meta-settings-private.h
index 619de1710..9f07940b7 100644
--- a/src/backends/meta-settings-private.h
+++ b/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 ())
diff --git a/src/backends/meta-settings.c b/src/backends/meta-settings.c
index 34d49b3ed..bc17969da 100644
--- a/src/backends/meta-settings.c
+++ b/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);
     }
diff --git a/src/backends/native/meta-backend-native.c b/src/backends/native/meta-backend-native.c
index c473681cb..6f21f9c2c 100644
--- a/src/backends/native/meta-backend-native.c
+++ b/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 *