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>

README.md

@@ -11,6 +11,8 @@ GameMode adjusts the nice priority of games to -4 by default to give it a slight
 
 Please take note that some games may actually run seemingly slower with `SCHED_ISO` if the game makes use of busy looping while interacting with the graphic driver. The same may happen if you apply too strong nice values. This effect is called priority inversion: Due to the high priority given to busy loops, there may be too few resources left for the graphics driver. Thus, sane defaults were chosen to not expose this effect on most systems. Part of this default is a heuristic which automatically turns off `SCHED_ISO` if GameMode detects three or less CPU cores. Your experience may change based on using GL threaded optimizations, CPU core binding (taskset), the graphic driver, or different CPU architectures. If you experience bad input latency or inconsistent FPS, try switching these configurations on or off first and report back. `SCHED_ISO` comes with a protection against this effect by falling back to normal scheduling as soon as the `SCHED_ISO` process uses more than 70% avarage across all CPU cores. This default value can be adjusted outside of the scope of GameMode (it's in `/proc/sys/kernel/iso_cpu`). This value also protects against compromising system stability, do not set it to 100% as this would turn the game into a full real time process, thus potentially starving all other OS components from CPU resources.
 
+GameMode can adjust the io priority of games to benefit from reduced lag and latency when a game has to load assets on demand.
+
 Issues with GameMode should be reported here in the issues section, and not reported to Feral directly.
 
 ---

daemon/gamemode.c

@@ -35,6 +35,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #include "daemon_config.h"
 #include "governors-query.h"
 #include "governors.h"
+#include "ioprio.h"
 #include "logging.h"
 
 #include <linux/limits.h>
@@ -241,6 +242,56 @@ static void game_mode_apply_scheduler(GameModeContext *self, pid_t client)
 	}
 }
 
+/**
+ * Apply io priorities
+ *
+ * This tries to change the io priority of the client to a value specified
+ * and can possibly reduce lags or latency when a game has to load assets
+ * on demand.
+ */
+static void game_mode_apply_ioprio(GameModeContext *self, pid_t client)
+{
+	LOG_MSG("Setting scheduling policies...\n");
+
+	/*
+	 * read configuration "ioprio" (0..7)
+	 */
+	int ioprio = 0;
+	config_get_ioprio_value(self->config, &ioprio);
+	if (IOPRIO_RESET_DEFAULT == ioprio) {
+		LOG_MSG("IO priority will be reset to default behavior (based on CPU priority).\n");
+		ioprio = 0;
+	} else if (IOPRIO_DONT_SET == ioprio) {
+		return;
+	} else {
+		/* maybe clamp the value */
+		int invalid_ioprio = ioprio;
+		ioprio = CLAMP(0, 7, ioprio);
+		if (ioprio != invalid_ioprio)
+			LOG_ERROR("IO priority value %d invalid, clamping to %d\n", invalid_ioprio, ioprio);
+
+		/* We support only IOPRIO_CLASS_BE as IOPRIO_CLASS_RT required CAP_SYS_ADMIN */
+		ioprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, ioprio);
+	}
+
+	/*
+	 * Actually apply the io priority
+	 */
+	int c = IOPRIO_PRIO_CLASS(ioprio), p = IOPRIO_PRIO_DATA(ioprio);
+	if (ioprio_set(IOPRIO_WHO_PROCESS, client, ioprio) == 0) {
+		if (0 == ioprio)
+			LOG_MSG("Resetting client [%d] IO priority.\n", client);
+		else
+			LOG_MSG("Setting client [%d] IO priority to (%d,%d).\n", client, c, p);
+	} else {
+		LOG_ERROR("Setting client [%d] IO priority to (%d,%d) failed with error %d, ignoring\n",
+		          client,
+		          c,
+		          p,
+		          errno);
+	}
+}
+
 /**
  * Pivot into game mode.
  *
@@ -437,6 +488,9 @@ bool game_mode_context_register(GameModeContext *self, pid_t client)
 	/* Apply scheduler policies */
 	game_mode_apply_scheduler(self, client);
 
+	/* Apply io priorities */
+	game_mode_apply_ioprio(self, client);
+
 	return true;
 error_cleanup:
 	game_mode_client_free(cl);

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);
+}