/******************************************************************************
   * 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/bootmem.h>
  #include <linux/swap.h>

  #include <linux/mm.h>
  #include <linux/mman.h>

  #include <linux/module.h>

  #include <linux/workqueue.h>

  #include <linux/device.h>

  #include <xen/balloon.h>

  #include <xen/tmem.h>

  #include <xen/xen.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;

  /*
   * Minimum usable RAM in MB for selfballooning target for balloon.
   * If non-zero, it is added to totalreserve_pages and self-ballooning
   * will not balloon below the sum.  If zero, a piecewise linear function
   * is calculated as a minimum and added to totalreserve_pages.  Note that
   * setting this value indiscriminately may cause OOMs and crashes.
   */
  static unsigned int selfballoon_min_usable_mb;

  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 */

  #define MB2PAGES(mb)	((mb) << (20 - PAGE_SHIFT))

  /*
   * 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, floor_pages;
  	unsigned long useful_pages;

  	bool reset_timer = false;
  
  	if (xen_selfballooning_enabled) {

  		cur_pages = totalram_pages;

  		tgt_pages = cur_pages; /* default is no change */
  		goal_pages = percpu_counter_read_positive(&vm_committed_as) +

  				totalreserve_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 */

  		useful_pages = max_pfn - totalreserve_pages;
  		if (selfballoon_min_usable_mb != 0)
  			floor_pages = totalreserve_pages +
  					MB2PAGES(selfballoon_min_usable_mb);
  		/* piecewise linear function ending in ~3% slope */
  		else if (useful_pages < MB2PAGES(16))
  			floor_pages = max_pfn; /* not worth ballooning */
  		else if (useful_pages < MB2PAGES(64))
  			floor_pages = totalreserve_pages + MB2PAGES(16) +
  					((useful_pages - MB2PAGES(16)) >> 1);
  		else if (useful_pages < MB2PAGES(512))
  			floor_pages = totalreserve_pages + MB2PAGES(40) +
  					((useful_pages - MB2PAGES(40)) >> 3);
  		else /* useful_pages >= MB2PAGES(512) */
  			floor_pages = totalreserve_pages + MB2PAGES(99) +
  					((useful_pages - MB2PAGES(99)) >> 5);
  		if (tgt_pages < floor_pages)
  			tgt_pages = floor_pages;
  		balloon_set_new_target(tgt_pages +
  			balloon_stats.current_pages - totalram_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/capability.h>
  
  #define SELFBALLOON_SHOW(name, format, args...)				\

  	static ssize_t show_##name(struct device *dev,	\
  					  struct device_attribute *attr, \
  					  char *buf) \

  	{ \
  		return sprintf(buf, format, ##args); \
  	}
  
  SELFBALLOON_SHOW(selfballooning, "%d
  ", xen_selfballooning_enabled);

  static ssize_t store_selfballooning(struct device *dev,
  			    struct device_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 DEVICE_ATTR(selfballooning, S_IRUGO | S_IWUSR,

  		   show_selfballooning, store_selfballooning);
  
  SELFBALLOON_SHOW(selfballoon_interval, "%d
  ", selfballoon_interval);

  static ssize_t store_selfballoon_interval(struct device *dev,
  					  struct device_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 DEVICE_ATTR(selfballoon_interval, S_IRUGO | S_IWUSR,

  		   show_selfballoon_interval, store_selfballoon_interval);
  
  SELFBALLOON_SHOW(selfballoon_downhys, "%d
  ", selfballoon_downhysteresis);

  static ssize_t store_selfballoon_downhys(struct device *dev,
  					 struct device_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 DEVICE_ATTR(selfballoon_downhysteresis, S_IRUGO | S_IWUSR,

  		   show_selfballoon_downhys, store_selfballoon_downhys);
  
  
  SELFBALLOON_SHOW(selfballoon_uphys, "%d
  ", selfballoon_uphysteresis);

  static ssize_t store_selfballoon_uphys(struct device *dev,
  				       struct device_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 DEVICE_ATTR(selfballoon_uphysteresis, S_IRUGO | S_IWUSR,

  		   show_selfballoon_uphys, store_selfballoon_uphys);

  SELFBALLOON_SHOW(selfballoon_min_usable_mb, "%d
  ",
  				selfballoon_min_usable_mb);

  static ssize_t store_selfballoon_min_usable_mb(struct device *dev,
  					       struct device_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_min_usable_mb = val;
  	return count;
  }

  static DEVICE_ATTR(selfballoon_min_usable_mb, S_IRUGO | S_IWUSR,

  		   show_selfballoon_min_usable_mb,
  		   store_selfballoon_min_usable_mb);

  #ifdef CONFIG_FRONTSWAP
  SELFBALLOON_SHOW(frontswap_selfshrinking, "%d
  ", frontswap_selfshrinking);

  static ssize_t store_frontswap_selfshrinking(struct device *dev,
  					     struct device_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 DEVICE_ATTR(frontswap_selfshrinking, S_IRUGO | S_IWUSR,

  		   show_frontswap_selfshrinking, store_frontswap_selfshrinking);
  
  SELFBALLOON_SHOW(frontswap_inertia, "%d
  ", frontswap_inertia);

  static ssize_t store_frontswap_inertia(struct device *dev,
  				       struct device_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 DEVICE_ATTR(frontswap_inertia, S_IRUGO | S_IWUSR,

  		   show_frontswap_inertia, store_frontswap_inertia);
  
  SELFBALLOON_SHOW(frontswap_hysteresis, "%d
  ", frontswap_hysteresis);

  static ssize_t store_frontswap_hysteresis(struct device *dev,
  					  struct device_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 DEVICE_ATTR(frontswap_hysteresis, S_IRUGO | S_IWUSR,

  		   show_frontswap_hysteresis, store_frontswap_hysteresis);
  
  #endif /* CONFIG_FRONTSWAP */
  
  static struct attribute *selfballoon_attrs[] = {

  	&dev_attr_selfballooning.attr,
  	&dev_attr_selfballoon_interval.attr,
  	&dev_attr_selfballoon_downhysteresis.attr,
  	&dev_attr_selfballoon_uphysteresis.attr,
  	&dev_attr_selfballoon_min_usable_mb.attr,

  #ifdef CONFIG_FRONTSWAP

  	&dev_attr_frontswap_selfshrinking.attr,
  	&dev_attr_frontswap_hysteresis.attr,
  	&dev_attr_frontswap_inertia.attr,

  #endif
  	NULL
  };
  
  static struct attribute_group selfballoon_group = {
  	.name = "selfballoon",
  	.attrs = selfballoon_attrs
  };
  #endif

  int register_xen_selfballooning(struct device *dev)

  {
  	int error = -1;
  
  #ifdef CONFIG_SYSFS

  	error = sysfs_create_group(&dev->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.
  ");
  		return -ENODEV;
  	}
  
  	xen_selfballooning_enabled = tmem_enabled && use_selfballooning;
  	if (xen_selfballooning_enabled) {
  		pr_info("xen/balloon: Initializing Xen "
  					"selfballooning driver.
  ");
  		enable = true;
  	}
  #ifdef CONFIG_FRONTSWAP
  	frontswap_selfshrinking = tmem_enabled && use_frontswap_selfshrink;
  	if (frontswap_selfshrinking) {
  		pr_info("xen/balloon: Initializing frontswap "
  					"selfshrinking driver.
  ");
  		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");