gamemode: Apply io priority to clients

This commit allows changing the io priority of the client to a value
specified in the configuration file. That can possibly reduce lags or
latency when a game has to load assets on demand and you have background
IO activity running (or other concurrent IO).

Signed-off-by: Kai Krakow <kai@kaishome.de>

daemon/ioprio.h

@@ -0,0 +1,82 @@
+/*
+
+Copyright (c) 2017-2018, Feral Interactive
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+ * Redistributions of source code must retain the above copyright notice,
+   this list of conditions and the following disclaimer.
+ * Redistributions in binary form must reproduce the above copyright
+   notice, this list of conditions and the following disclaimer in the
+   documentation and/or other materials provided with the distribution.
+ * Neither the name of Feral Interactive nor the names of its contributors
+   may be used to endorse or promote products derived from this software
+   without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGE.
+
+ */
+
+#pragma once
+
+#define _GNU_SOURCE
+#include <sys/syscall.h>
+#include <unistd.h>
+
+/**
+ * Define the syscall interface in Linux because it is missing from glibc
+ */
+
+#ifndef IOPRIO_BITS
+#define IOPRIO_BITS (16)
+#endif
+
+#ifndef IOPRIO_CLASS_SHIFT
+#define IOPRIO_CLASS_SHIFT (13)
+#endif
+
+#ifndef IOPRIO_PRIO_MASK
+#define IOPRIO_PRIO_MASK ((1UL << IOPRIO_CLASS_SHIFT) - 1)
+#endif
+
+#ifndef IOPRIO_PRIO_CLASS
+#define IOPRIO_PRIO_CLASS(mask) ((mask) >> IOPRIO_CLASS_SHIFT)
+#endif
+
+#ifndef IOPRIO_PRIO_DATA
+#define IOPRIO_PRIO_DATA(mask) ((mask)&IOPRIO_PRIO_MASK)
+#endif
+
+#ifndef IOPRIO_PRIO_VALUE
+#define IOPRIO_PRIO_VALUE(class, data) (((class) << IOPRIO_CLASS_SHIFT) | data)
+#endif
+
+enum {
+	IOPRIO_CLASS_NONE,
+	IOPRIO_CLASS_RT,
+	IOPRIO_CLASS_BE,
+	IOPRIO_CLASS_IDLE,
+};
+
+enum {
+	IOPRIO_WHO_PROCESS = 1,
+	IOPRIO_WHO_PGRP,
+	IOPRIO_WHO_USER,
+};
+
+static inline int ioprio_set(int which, int who, int ioprio)
+{
+	return (int)syscall(SYS_ioprio_set, which, who, ioprio);
+}