xen: tmem: self-ballooning and frontswap-selfshrinking

This patch introduces two in-kernel drivers for Xen transcendent memory
("tmem") functionality that complement cleancache and frontswap.  Both
use control theory to dynamically adjust and optimize memory utilization.
Selfballooning controls the in-kernel Xen balloon driver, targeting a goal
value (vm_committed_as), thus pushing less frequently used clean
page cache pages (through the cleancache code) into Xen tmem where
Xen can balance needs across all VMs residing on the physical machine.
Frontswap-selfshrinking controls the number of pages in frontswap,
driving it towards zero (effectively doing a partial swapoff) when
in-kernel memory pressure subsides, freeing up RAM for other VMs.

More detail is provided in the header comment of xen-selfballooning.c.

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>

[v8: konrad.wilk@oracle.com: set default enablement depending on frontswap]
[v7: konrad.wilk@oracle.com: fix capitalization and punctuation in comments]
[v6: fix frontswap-selfshrinking initialization]
[v6: konrad.wilk@oracle.com: fix init pr_infos; add comments about swap]
[v5: konrad.wilk@oracle.com: add NULL to attr list; move inits up to decls]
[v4: dkiper@net-space.pl: use strict_strtoul plus a few syntactic nits]
[v3: konrad.wilk@oracle.com: fix potential divides-by-zero]
[v3: konrad.wilk@oracle.com: add many more comments, fix nits]
[v2: rebased to linux-3.0-rc1]
[v2: Ian.Campbell@citrix.com: reorganize as new file (xen-selfballoon.c)]
[v2: dkiper@net-space.pl: proper access to vm_committed_as]
[v2: dkiper@net-space.pl: accounting fixes]
Cc: Jan Beulich <JBeulich@novell.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: <xen-devel@lists.xensource.com>

drivers/xen/Kconfig

@@ -9,6 +9,23 @@ config XEN_BALLOON
 	  the system to expand the domain's memory allocation, or alternatively
 	  return unneeded memory to the system.
 
+config XEN_SELFBALLOONING
+	bool "Dynamically self-balloon kernel memory to target"
+	depends on XEN && XEN_BALLOON && CLEANCACHE && SWAP
+	default n
+	help
+	  Self-ballooning dynamically balloons available kernel memory driven
+	  by the current usage of anonymous memory ("committed AS") and
+	  controlled by various sysfs-settable parameters.  Configuring
+	  FRONTSWAP is highly recommended; if it is not configured, self-
+	  ballooning is disabled by default but can be enabled with the
+	  'selfballooning' kernel boot parameter.  If FRONTSWAP is configured,
+	  frontswap-selfshrinking is enabled by default but can be disabled
+	  with the 'noselfshrink' kernel boot parameter; and self-ballooning
+	  is enabled by default but can be disabled with the 'noselfballooning'
+	  kernel boot parameter.  Note that systems without a sufficiently
+	  large swap device should not enable self-ballooning.
+
 config XEN_SCRUB_PAGES
 	bool "Scrub pages before returning them to system"
 	depends on XEN_BALLOON

drivers/xen/Makefile

@@ -9,6 +9,7 @@ obj-$(CONFIG_BLOCK)			+= biomerge.o
 obj-$(CONFIG_HOTPLUG_CPU)		+= cpu_hotplug.o
 obj-$(CONFIG_XEN_XENCOMM)		+= xencomm.o
 obj-$(CONFIG_XEN_BALLOON)		+= xen-balloon.o
+obj-$(CONFIG_XEN_SELFBALLOONING)	+= xen-selfballoon.o
 obj-$(CONFIG_XEN_DEV_EVTCHN)		+= xen-evtchn.o
 obj-$(CONFIG_XEN_GNTDEV)		+= xen-gntdev.o
 obj-$(CONFIG_XEN_GRANT_DEV_ALLOC)	+= xen-gntalloc.o

drivers/xen/xen-balloon.c

@@ -98,6 +98,8 @@ static int __init balloon_init(void)
 
 	register_balloon(&balloon_sysdev);
 
+	register_xen_selfballooning(&balloon_sysdev);
+
 	target_watch.callback = watch_target;
 	xenstore_notifier.notifier_call = balloon_init_watcher;
 

drivers/xen/xen-selfballoon.c

@@ -0,0 +1,485 @@
+/******************************************************************************
+ * Xen selfballoon driver (and optional frontswap self-shrinking driver)
+ *
+ * Copyright (c) 2009-2011, Dan Magenheimer, Oracle Corp.
+ *
+ * This code complements the cleancache and frontswap patchsets to optimize
+ * support for Xen Transcendent Memory ("tmem").  The policy it implements
+ * is rudimentary and will likely improve over time, but it does work well
+ * enough today.
+ *
+ * Two functionalities are implemented here which both use "control theory"
+ * (feedback) to optimize memory utilization. In a virtualized environment
+ * such as Xen, RAM is often a scarce resource and we would like to ensure
+ * that each of a possibly large number of virtual machines is using RAM
+ * efficiently, i.e. using as little as possible when under light load
+ * and obtaining as much as possible when memory demands are high.
+ * Since RAM needs vary highly dynamically and sometimes dramatically,
+ * "hysteresis" is used, that is, memory target is determined not just
+ * on current data but also on past data stored in the system.
+ *
+ * "Selfballooning" creates memory pressure by managing the Xen balloon
+ * driver to decrease and increase available kernel memory, driven
+ * largely by the target value of "Committed_AS" (see /proc/meminfo).
+ * Since Committed_AS does not account for clean mapped pages (i.e. pages
+ * in RAM that are identical to pages on disk), selfballooning has the
+ * affect of pushing less frequently used clean pagecache pages out of
+ * kernel RAM and, presumably using cleancache, into Xen tmem where
+ * Xen can more efficiently optimize RAM utilization for such pages.
+ *
+ * When kernel memory demand unexpectedly increases faster than Xen, via
+ * the selfballoon driver, is able to (or chooses to) provide usable RAM,
+ * the kernel may invoke swapping.  In most cases, frontswap is able
+ * to absorb this swapping into Xen tmem.  However, due to the fact
+ * that the kernel swap subsystem assumes swapping occurs to a disk,
+ * swapped pages may sit on the disk for a very long time; even if
+ * the kernel knows the page will never be used again.  This is because
+ * the disk space costs very little and can be overwritten when
+ * necessary.  When such stale pages are in frontswap, however, they
+ * are taking up valuable real estate.  "Frontswap selfshrinking" works
+ * to resolve this:  When frontswap activity is otherwise stable
+ * and the guest kernel is not under memory pressure, the "frontswap
+ * selfshrinking" accounts for this by providing pressure to remove some
+ * pages from frontswap and return them to kernel memory.
+ *
+ * For both "selfballooning" and "frontswap-selfshrinking", a worker
+ * thread is used and sysfs tunables are provided to adjust the frequency
+ * and rate of adjustments to achieve the goal, as well as to disable one
+ * or both functions independently.
+ *
+ * While some argue that this functionality can and should be implemented
+ * in userspace, it has been observed that bad things happen (e.g. OOMs).
+ *
+ * System configuration note: Selfballooning should not be enabled on
+ * systems without a sufficiently large swap device configured; for best
+ * results, it is recommended that total swap be increased by the size
+ * of the guest memory.  Also, while technically not required to be
+ * configured, it is highly recommended that frontswap also be configured
+ * and enabled when selfballooning is running.  So, selfballooning
+ * is disabled by default if frontswap is not configured and can only
+ * be enabled with the "selfballooning" kernel boot option; similarly
+ * selfballooning is enabled by default if frontswap is configured and
+ * can be disabled with the "noselfballooning" kernel boot option.  Finally,
+ * when frontswap is configured, frontswap-selfshrinking can be disabled
+ * with the "noselfshrink" kernel boot option.
+ *
+ * Selfballooning is disallowed in domain0 and force-disabled.
+ *
+ */
+
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/mman.h>
+
+#include <xen/balloon.h>
+
+#include <xen/tmem.h>
+
+/* Enable/disable with sysfs. */
+static int xen_selfballooning_enabled __read_mostly;
+
+/*
+ * Controls rate at which memory target (this iteration) approaches
+ * ultimate goal when memory need is increasing (up-hysteresis) or
+ * decreasing (down-hysteresis). Higher values of hysteresis cause
+ * slower increases/decreases. The default values for the various
+ * parameters were deemed reasonable by experimentation, may be
+ * workload-dependent, and can all be adjusted via sysfs.
+ */
+static unsigned int selfballoon_downhysteresis __read_mostly = 8;
+static unsigned int selfballoon_uphysteresis __read_mostly = 1;
+
+/* In HZ, controls frequency of worker invocation. */
+static unsigned int selfballoon_interval __read_mostly = 5;
+
+static void selfballoon_process(struct work_struct *work);
+static DECLARE_DELAYED_WORK(selfballoon_worker, selfballoon_process);
+
+#ifdef CONFIG_FRONTSWAP
+#include <linux/frontswap.h>
+
+/* Enable/disable with sysfs. */
+static bool frontswap_selfshrinking __read_mostly;
+
+/* Enable/disable with kernel boot option. */
+static bool use_frontswap_selfshrink __initdata = true;
+
+/*
+ * The default values for the following parameters were deemed reasonable
+ * by experimentation, may be workload-dependent, and can all be
+ * adjusted via sysfs.
+ */
+
+/* Control rate for frontswap shrinking. Higher hysteresis is slower. */
+static unsigned int frontswap_hysteresis __read_mostly = 20;
+
+/*
+ * Number of selfballoon worker invocations to wait before observing that
+ * frontswap selfshrinking should commence. Note that selfshrinking does
+ * not use a separate worker thread.
+ */
+static unsigned int frontswap_inertia __read_mostly = 3;
+
+/* Countdown to next invocation of frontswap_shrink() */
+static unsigned long frontswap_inertia_counter;
+
+/*
+ * Invoked by the selfballoon worker thread, uses current number of pages
+ * in frontswap (frontswap_curr_pages()), previous status, and control
+ * values (hysteresis and inertia) to determine if frontswap should be
+ * shrunk and what the new frontswap size should be.  Note that
+ * frontswap_shrink is essentially a partial swapoff that immediately
+ * transfers pages from the "swap device" (frontswap) back into kernel
+ * RAM; despite the name, frontswap "shrinking" is very different from
+ * the "shrinker" interface used by the kernel MM subsystem to reclaim
+ * memory.
+ */
+static void frontswap_selfshrink(void)
+{
+	static unsigned long cur_frontswap_pages;
+	static unsigned long last_frontswap_pages;
+	static unsigned long tgt_frontswap_pages;
+
+	last_frontswap_pages = cur_frontswap_pages;
+	cur_frontswap_pages = frontswap_curr_pages();
+	if (!cur_frontswap_pages ||
+			(cur_frontswap_pages > last_frontswap_pages)) {
+		frontswap_inertia_counter = frontswap_inertia;
+		return;
+	}
+	if (frontswap_inertia_counter && --frontswap_inertia_counter)
+		return;
+	if (cur_frontswap_pages <= frontswap_hysteresis)
+		tgt_frontswap_pages = 0;
+	else
+		tgt_frontswap_pages = cur_frontswap_pages -
+			(cur_frontswap_pages / frontswap_hysteresis);
+	frontswap_shrink(tgt_frontswap_pages);
+}
+
+static int __init xen_nofrontswap_selfshrink_setup(char *s)
+{
+	use_frontswap_selfshrink = false;
+	return 1;
+}
+
+__setup("noselfshrink", xen_nofrontswap_selfshrink_setup);
+
+/* Disable with kernel boot option. */
+static bool use_selfballooning __initdata = true;
+
+static int __init xen_noselfballooning_setup(char *s)
+{
+	use_selfballooning = false;
+	return 1;
+}
+
+__setup("noselfballooning", xen_noselfballooning_setup);
+#else /* !CONFIG_FRONTSWAP */
+/* Enable with kernel boot option. */
+static bool use_selfballooning __initdata = false;
+
+static int __init xen_selfballooning_setup(char *s)
+{
+	use_selfballooning = true;
+	return 1;
+}
+
+__setup("selfballooning", xen_selfballooning_setup);
+#endif /* CONFIG_FRONTSWAP */
+
+/*
+ * Use current balloon size, the goal (vm_committed_as), and hysteresis
+ * parameters to set a new target balloon size
+ */
+static void selfballoon_process(struct work_struct *work)
+{
+	unsigned long cur_pages, goal_pages, tgt_pages;
+	bool reset_timer = false;
+
+	if (xen_selfballooning_enabled) {
+		cur_pages = balloon_stats.current_pages;
+		tgt_pages = cur_pages; /* default is no change */
+		goal_pages = percpu_counter_read_positive(&vm_committed_as) +
+			balloon_stats.current_pages - totalram_pages;
+#ifdef CONFIG_FRONTSWAP
+		/* allow space for frontswap pages to be repatriated */
+		if (frontswap_selfshrinking && frontswap_enabled)
+			goal_pages += frontswap_curr_pages();
+#endif
+		if (cur_pages > goal_pages)
+			tgt_pages = cur_pages -
+				((cur_pages - goal_pages) /
+				  selfballoon_downhysteresis);
+		else if (cur_pages < goal_pages)
+			tgt_pages = cur_pages +
+				((goal_pages - cur_pages) /
+				  selfballoon_uphysteresis);
+		/* else if cur_pages == goal_pages, no change */
+		balloon_set_new_target(tgt_pages);
+		reset_timer = true;
+	}
+#ifdef CONFIG_FRONTSWAP
+	if (frontswap_selfshrinking && frontswap_enabled) {
+		frontswap_selfshrink();
+		reset_timer = true;
+	}
+#endif
+	if (reset_timer)
+		schedule_delayed_work(&selfballoon_worker,
+			selfballoon_interval * HZ);
+}
+
+#ifdef CONFIG_SYSFS
+
+#include <linux/sysdev.h>
+#include <linux/capability.h>
+
+#define SELFBALLOON_SHOW(name, format, args...)				\
+	static ssize_t show_##name(struct sys_device *dev,	\
+					   struct sysdev_attribute *attr, \
+					   char *buf) \
+	{ \
+		return sprintf(buf, format, ##args); \
+	}
+
+SELFBALLOON_SHOW(selfballooning, "%d\n", xen_selfballooning_enabled);
+
+static ssize_t store_selfballooning(struct sys_device *dev,
+			    struct sysdev_attribute *attr,
+			    const char *buf,
+			    size_t count)
+{
+	bool was_enabled = xen_selfballooning_enabled;
+	unsigned long tmp;
+	int err;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	err = strict_strtoul(buf, 10, &tmp);
+	if (err || ((tmp != 0) && (tmp != 1)))
+		return -EINVAL;
+
+	xen_selfballooning_enabled = !!tmp;
+	if (!was_enabled && xen_selfballooning_enabled)
+		schedule_delayed_work(&selfballoon_worker,
+			selfballoon_interval * HZ);
+
+	return count;
+}
+
+static SYSDEV_ATTR(selfballooning, S_IRUGO | S_IWUSR,
+		   show_selfballooning, store_selfballooning);
+
+SELFBALLOON_SHOW(selfballoon_interval, "%d\n", selfballoon_interval);
+
+static ssize_t store_selfballoon_interval(struct sys_device *dev,
+					  struct sysdev_attribute *attr,
+					  const char *buf,
+					  size_t count)
+{
+	unsigned long val;
+	int err;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+	err = strict_strtoul(buf, 10, &val);
+	if (err || val == 0)
+		return -EINVAL;
+	selfballoon_interval = val;
+	return count;
+}
+
+static SYSDEV_ATTR(selfballoon_interval, S_IRUGO | S_IWUSR,
+		   show_selfballoon_interval, store_selfballoon_interval);
+
+SELFBALLOON_SHOW(selfballoon_downhys, "%d\n", selfballoon_downhysteresis);
+
+static ssize_t store_selfballoon_downhys(struct sys_device *dev,
+					 struct sysdev_attribute *attr,
+					 const char *buf,
+					 size_t count)
+{
+	unsigned long val;
+	int err;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+	err = strict_strtoul(buf, 10, &val);
+	if (err || val == 0)
+		return -EINVAL;
+	selfballoon_downhysteresis = val;
+	return count;
+}
+
+static SYSDEV_ATTR(selfballoon_downhysteresis, S_IRUGO | S_IWUSR,
+		   show_selfballoon_downhys, store_selfballoon_downhys);
+
+
+SELFBALLOON_SHOW(selfballoon_uphys, "%d\n", selfballoon_uphysteresis);
+
+static ssize_t store_selfballoon_uphys(struct sys_device *dev,
+				       struct sysdev_attribute *attr,
+				       const char *buf,
+				       size_t count)
+{
+	unsigned long val;
+	int err;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+	err = strict_strtoul(buf, 10, &val);
+	if (err || val == 0)
+		return -EINVAL;
+	selfballoon_uphysteresis = val;
+	return count;
+}
+
+static SYSDEV_ATTR(selfballoon_uphysteresis, S_IRUGO | S_IWUSR,
+		   show_selfballoon_uphys, store_selfballoon_uphys);
+
+#ifdef CONFIG_FRONTSWAP
+SELFBALLOON_SHOW(frontswap_selfshrinking, "%d\n", frontswap_selfshrinking);
+
+static ssize_t store_frontswap_selfshrinking(struct sys_device *dev,
+					     struct sysdev_attribute *attr,
+					     const char *buf,
+					     size_t count)
+{
+	bool was_enabled = frontswap_selfshrinking;
+	unsigned long tmp;
+	int err;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+	err = strict_strtoul(buf, 10, &tmp);
+	if (err || ((tmp != 0) && (tmp != 1)))
+		return -EINVAL;
+	frontswap_selfshrinking = !!tmp;
+	if (!was_enabled && !xen_selfballooning_enabled &&
+	     frontswap_selfshrinking)
+		schedule_delayed_work(&selfballoon_worker,
+			selfballoon_interval * HZ);
+
+	return count;
+}
+
+static SYSDEV_ATTR(frontswap_selfshrinking, S_IRUGO | S_IWUSR,
+		   show_frontswap_selfshrinking, store_frontswap_selfshrinking);
+
+SELFBALLOON_SHOW(frontswap_inertia, "%d\n", frontswap_inertia);
+
+static ssize_t store_frontswap_inertia(struct sys_device *dev,
+				       struct sysdev_attribute *attr,
+				       const char *buf,
+				       size_t count)
+{
+	unsigned long val;
+	int err;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+	err = strict_strtoul(buf, 10, &val);
+	if (err || val == 0)
+		return -EINVAL;
+	frontswap_inertia = val;
+	frontswap_inertia_counter = val;
+	return count;
+}
+
+static SYSDEV_ATTR(frontswap_inertia, S_IRUGO | S_IWUSR,
+		   show_frontswap_inertia, store_frontswap_inertia);
+
+SELFBALLOON_SHOW(frontswap_hysteresis, "%d\n", frontswap_hysteresis);
+
+static ssize_t store_frontswap_hysteresis(struct sys_device *dev,
+					  struct sysdev_attribute *attr,
+					  const char *buf,
+					  size_t count)
+{
+	unsigned long val;
+	int err;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+	err = strict_strtoul(buf, 10, &val);
+	if (err || val == 0)
+		return -EINVAL;
+	frontswap_hysteresis = val;
+	return count;
+}
+
+static SYSDEV_ATTR(frontswap_hysteresis, S_IRUGO | S_IWUSR,
+		   show_frontswap_hysteresis, store_frontswap_hysteresis);
+
+#endif /* CONFIG_FRONTSWAP */
+
+static struct attribute *selfballoon_attrs[] = {
+	&attr_selfballooning.attr,
+	&attr_selfballoon_interval.attr,
+	&attr_selfballoon_downhysteresis.attr,
+	&attr_selfballoon_uphysteresis.attr,
+#ifdef CONFIG_FRONTSWAP
+	&attr_frontswap_selfshrinking.attr,
+	&attr_frontswap_hysteresis.attr,
+	&attr_frontswap_inertia.attr,
+#endif
+	NULL
+};
+
+static struct attribute_group selfballoon_group = {
+	.name = "selfballoon",
+	.attrs = selfballoon_attrs
+};
+#endif
+
+int register_xen_selfballooning(struct sys_device *sysdev)
+{
+	int error = -1;
+
+#ifdef CONFIG_SYSFS
+	error = sysfs_create_group(&sysdev->kobj, &selfballoon_group);
+#endif
+	return error;
+}
+EXPORT_SYMBOL(register_xen_selfballooning);
+
+static int __init xen_selfballoon_init(void)
+{
+	bool enable = false;
+
+	if (!xen_domain())
+		return -ENODEV;
+
+	if (xen_initial_domain()) {
+		pr_info("xen/balloon: Xen selfballooning driver "
+				"disabled for domain0.\n");
+		return -ENODEV;
+	}
+
+	xen_selfballooning_enabled = tmem_enabled && use_selfballooning;
+	if (xen_selfballooning_enabled) {
+		pr_info("xen/balloon: Initializing Xen "
+					"selfballooning driver.\n");
+		enable = true;
+	}
+#ifdef CONFIG_FRONTSWAP
+	frontswap_selfshrinking = tmem_enabled && use_frontswap_selfshrink;
+	if (frontswap_selfshrinking) {
+		pr_info("xen/balloon: Initializing frontswap "
+					"selfshrinking driver.\n");
+		enable = true;
+	}
+#endif
+	if (!enable)
+		return -ENODEV;
+
+	schedule_delayed_work(&selfballoon_worker, selfballoon_interval * HZ);
+
+	return 0;
+}
+
+subsys_initcall(xen_selfballoon_init);
+
+MODULE_LICENSE("GPL");

include/xen/balloon.h

@@ -23,3 +23,13 @@ void balloon_set_new_target(unsigned long target);
 
 int alloc_xenballooned_pages(int nr_pages, struct page** pages);
 void free_xenballooned_pages(int nr_pages, struct page** pages);
+
+struct sys_device;
+#ifdef CONFIG_XEN_SELFBALLOONING
+extern int register_xen_selfballooning(struct sys_device *sysdev);
+#else
+static inline int register_xen_selfballooning(struct sys_device *sysdev)
+{
+	return -ENOSYS;
+}
+#endif

include/xen/tmem.h

@@ -0,0 +1,5 @@
+#ifndef _XEN_TMEM_H
+#define _XEN_TMEM_H
+/* defined in drivers/xen/tmem.c */
+extern int tmem_enabled;
+#endif /* _XEN_TMEM_H */