wayland: Make setting realtime scheduler more fine grained

With this only the main and the libinput threads will use realtime
scheduling, so it will be harder to leak realtime scheduling to somebody
else.

The only caveat is that kwin would need to keep CAP_SYS_NICE around,
however on the other hand, it's needed to ensure that kwin_wayland is
able to get high priority EGL contexts with some drivers, e.g. intel.

src/CMakeLists.txt

@@ -302,9 +302,6 @@ target_link_libraries(kwin_wayland
     KWinXwaylandServerModule
 )
 kcoreaddons_target_static_plugins(kwin_wayland "kwin/effects/plugins")
-if (HAVE_LIBCAP)
-    target_link_libraries(kwin_wayland ${Libcap_LIBRARIES})
-endif()
 
 install(TARGETS kwin_wayland ${KDE_INSTALL_TARGETS_DEFAULT_ARGS})
 if (HAVE_LIBCAP)

src/backends/libinput/connection.cpp

@@ -24,6 +24,7 @@
 #include "libinput_logging.h"
 #include "session.h"
 #include "udev.h"
+#include "utils/realtime.h"
 
 #include <QDBusConnection>
 #include <QMutexLocker>
@@ -150,6 +151,9 @@ void Connection::setup()
 void Connection::doSetup()
 {
     Q_ASSERT(!m_notifier);
+
+    gainRealTime();
+
     m_notifier = new QSocketNotifier(m_input->fileDescriptor(), QSocketNotifier::Read, this);
     connect(m_notifier, &QSocketNotifier::activated, this, &Connection::handleEvent);
 

src/config-kwin.h.cmake

@@ -17,7 +17,6 @@
 #cmakedefine01 HAVE_GBM_BO_GET_FD_FOR_PLANE
 #cmakedefine01 HAVE_WAYLAND_EGL
 #cmakedefine01 HAVE_BREEZE_DECO
-#cmakedefine01 HAVE_LIBCAP
 #cmakedefine01 HAVE_SCHED_RESET_ON_FORK
 #cmakedefine01 HAVE_ACCESSIBILITY
 #if HAVE_BREEZE_DECO

src/main_wayland.cpp

@@ -15,6 +15,7 @@
 #include "inputmethod.h"
 #include "platform.h"
 #include "tabletmodemanager.h"
+#include "utils/realtime.h"
 #include "wayland/display.h"
 #include "wayland/seat_interface.h"
 #include "wayland_server.h"
@@ -38,10 +39,6 @@
 #include <QWindow>
 #include <qplatformdefs.h>
 
-#if HAVE_LIBCAP
-#include <sys/capability.h>
-#endif
-
 #include <sched.h>
 #include <sys/resource.h>
 
@@ -102,30 +99,6 @@ void disableDrKonqi()
 // that would enable drkonqi
 Q_CONSTRUCTOR_FUNCTION(disableDrKonqi)
 
-enum class RealTimeFlags {
-    DontReset,
-    ResetOnFork
-};
-
-namespace
-{
-void gainRealTime(RealTimeFlags flags = RealTimeFlags::DontReset)
-{
-#if HAVE_SCHED_RESET_ON_FORK
-    const int minPriority = sched_get_priority_min(SCHED_RR);
-    struct sched_param sp;
-    sp.sched_priority = minPriority;
-    int policy = SCHED_RR;
-    if (flags == RealTimeFlags::ResetOnFork) {
-        policy |= SCHED_RESET_ON_FORK;
-    }
-    sched_setscheduler(0, policy, &sp);
-#else
-    Q_UNUSED(flags);
-#endif
-}
-}
-
 //************************************
 // ApplicationWayland
 //************************************
@@ -170,11 +143,7 @@ void ApplicationWayland::performStartup()
     waylandServer()->initPlatform();
     createColorManager();
 
-    // try creating the Wayland Backend
     createInput();
-    // now libinput thread has been created, adjust scheduler to not leak into other processes
-    gainRealTime(RealTimeFlags::ResetOnFork);
-
     createInputMethod();
     TabletModeManager::create(this);
     createPlugins();
@@ -312,27 +281,6 @@ static QString automaticBackendSelection()
     return s_drmPlugin;
 }
 
-void dropNiceCapability()
-{
-#if HAVE_LIBCAP
-    cap_t caps = cap_get_proc();
-    if (!caps) {
-        return;
-    }
-    cap_value_t capList[] = {CAP_SYS_NICE};
-    if (cap_set_flag(caps, CAP_PERMITTED, 1, capList, CAP_CLEAR) == -1) {
-        cap_free(caps);
-        return;
-    }
-    if (cap_set_flag(caps, CAP_EFFECTIVE, 1, capList, CAP_CLEAR) == -1) {
-        cap_free(caps);
-        return;
-    }
-    cap_set_proc(caps);
-    cap_free(caps);
-#endif
-}
-
 } // namespace
 
 int main(int argc, char *argv[])
@@ -340,7 +288,6 @@ int main(int argc, char *argv[])
     KWin::Application::setupMalloc();
     KWin::Application::setupLocalizedString();
     KWin::gainRealTime();
-    KWin::dropNiceCapability();
 
     if (signal(SIGTERM, KWin::sighandler) == SIG_IGN) {
         signal(SIGTERM, SIG_IGN);

src/utils/CMakeLists.txt

@@ -2,6 +2,7 @@ target_sources(kwin PRIVATE
     abstract_opengl_context_attribute_builder.cpp
     common.cpp
     egl_context_attribute_builder.cpp
+    realtime.cpp
     subsurfacemonitor.cpp
     xcbutils.cpp
 )

src/utils/realtime.cpp

@@ -0,0 +1,26 @@
+/*
+    SPDX-FileCopyrightText: 2022 Vlad Zahorodnii <vlad.zahorodnii@kde.org>
+
+    SPDX-License-Identifier: GPL-2.0-or-later
+*/
+
+#include "realtime.h"
+
+#include "config-kwin.h"
+
+#include <sched.h>
+
+namespace KWin
+{
+
+void gainRealTime()
+{
+#if HAVE_SCHED_RESET_ON_FORK
+    const int minPriority = sched_get_priority_min(SCHED_RR);
+    sched_param sp;
+    sp.sched_priority = minPriority;
+    sched_setscheduler(0, SCHED_RR | SCHED_RESET_ON_FORK, &sp);
+#endif
+}
+
+} // namespace KWin

src/utils/realtime.h

@@ -0,0 +1,19 @@
+/*
+    SPDX-FileCopyrightText: 2022 Vlad Zahorodnii <vlad.zahorodnii@kde.org>
+
+    SPDX-License-Identifier: GPL-2.0-or-later
+*/
+
+#pragma once
+
+#include "kwin_export.h"
+
+namespace KWin
+{
+
+/**
+ * Makes the calling thread to use realtime scheduling.
+ */
+KWIN_EXPORT void gainRealTime();
+
+} // namespace KWin