/*
   * Linux VM pressure
   *
   * Copyright 2012 Linaro Ltd.
   *		  Anton Vorontsov <anton.vorontsov@linaro.org>
   *
   * Based on ideas from Andrew Morton, David Rientjes, KOSAKI Motohiro,
   * Leonid Moiseichuk, Mel Gorman, Minchan Kim and Pekka Enberg.
   *
   * This program is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 as published
   * by the Free Software Foundation.
   */
  
  #include <linux/cgroup.h>
  #include <linux/fs.h>
  #include <linux/log2.h>
  #include <linux/sched.h>
  #include <linux/mm.h>
  #include <linux/vmstat.h>
  #include <linux/eventfd.h>
  #include <linux/slab.h>
  #include <linux/swap.h>
  #include <linux/printk.h>
  #include <linux/vmpressure.h>
  
  /*
   * The window size (vmpressure_win) is the number of scanned pages before
   * we try to analyze scanned/reclaimed ratio. So the window is used as a
   * rate-limit tunable for the "low" level notification, and also for
   * averaging the ratio for medium/critical levels. Using small window
   * sizes can cause lot of false positives, but too big window size will
   * delay the notifications.
   *
   * As the vmscan reclaimer logic works with chunks which are multiple of
   * SWAP_CLUSTER_MAX, it makes sense to use it for the window size as well.
   *
   * TODO: Make the window size depend on machine size, as we do for vmstat
   * thresholds. Currently we set it to 512 pages (2MB for 4KB pages).
   */
  static const unsigned long vmpressure_win = SWAP_CLUSTER_MAX * 16;
  
  /*
   * These thresholds are used when we account memory pressure through
   * scanned/reclaimed ratio. The current values were chosen empirically. In
   * essence, they are percents: the higher the value, the more number
   * unsuccessful reclaims there were.
   */
  static const unsigned int vmpressure_level_med = 60;
  static const unsigned int vmpressure_level_critical = 95;
  
  /*
   * When there are too little pages left to scan, vmpressure() may miss the
   * critical pressure as number of pages will be less than "window size".
   * However, in that case the vmscan priority will raise fast as the
   * reclaimer will try to scan LRUs more deeply.
   *
   * The vmscan logic considers these special priorities:
   *
   * prio == DEF_PRIORITY (12): reclaimer starts with that value
   * prio <= DEF_PRIORITY - 2 : kswapd becomes somewhat overwhelmed
   * prio == 0                : close to OOM, kernel scans every page in an lru
   *
   * Any value in this range is acceptable for this tunable (i.e. from 12 to
   * 0). Current value for the vmpressure_level_critical_prio is chosen
   * empirically, but the number, in essence, means that we consider
   * critical level when scanning depth is ~10% of the lru size (vmscan
   * scans 'lru_size >> prio' pages, so it is actually 12.5%, or one
   * eights).
   */
  static const unsigned int vmpressure_level_critical_prio = ilog2(100 / 10);
  
  static struct vmpressure *work_to_vmpressure(struct work_struct *work)
  {
  	return container_of(work, struct vmpressure, work);
  }
  
  static struct vmpressure *vmpressure_parent(struct vmpressure *vmpr)
  {
  	struct cgroup_subsys_state *css = vmpressure_to_css(vmpr);
  	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
  
  	memcg = parent_mem_cgroup(memcg);
  	if (!memcg)
  		return NULL;
  	return memcg_to_vmpressure(memcg);
  }
  
  enum vmpressure_levels {
  	VMPRESSURE_LOW = 0,
  	VMPRESSURE_MEDIUM,
  	VMPRESSURE_CRITICAL,
  	VMPRESSURE_NUM_LEVELS,
  };
  
  static const char * const vmpressure_str_levels[] = {
  	[VMPRESSURE_LOW] = "low",
  	[VMPRESSURE_MEDIUM] = "medium",
  	[VMPRESSURE_CRITICAL] = "critical",
  };
  
  static enum vmpressure_levels vmpressure_level(unsigned long pressure)
  {
  	if (pressure >= vmpressure_level_critical)
  		return VMPRESSURE_CRITICAL;
  	else if (pressure >= vmpressure_level_med)
  		return VMPRESSURE_MEDIUM;
  	return VMPRESSURE_LOW;
  }
  
  static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
  						    unsigned long reclaimed)
  {
  	unsigned long scale = scanned + reclaimed;
  	unsigned long pressure;
  
  	/*
  	 * We calculate the ratio (in percents) of how many pages were
  	 * scanned vs. reclaimed in a given time frame (window). Note that
  	 * time is in VM reclaimer's "ticks", i.e. number of pages
  	 * scanned. This makes it possible to set desired reaction time
  	 * and serves as a ratelimit.
  	 */
  	pressure = scale - (reclaimed * scale / scanned);
  	pressure = pressure * 100 / scale;
  
  	pr_debug("%s: %3lu  (s: %lu  r: %lu)
  ", __func__, pressure,
  		 scanned, reclaimed);
  
  	return vmpressure_level(pressure);
  }
  
  struct vmpressure_event {
  	struct eventfd_ctx *efd;
  	enum vmpressure_levels level;
  	struct list_head node;
  };
  
  static bool vmpressure_event(struct vmpressure *vmpr,
  			     unsigned long scanned, unsigned long reclaimed)
  {
  	struct vmpressure_event *ev;
  	enum vmpressure_levels level;
  	bool signalled = false;
  
  	level = vmpressure_calc_level(scanned, reclaimed);
  
  	mutex_lock(&vmpr->events_lock);
  
  	list_for_each_entry(ev, &vmpr->events, node) {
  		if (level >= ev->level) {
  			eventfd_signal(ev->efd, 1);
  			signalled = true;
  		}
  	}
  
  	mutex_unlock(&vmpr->events_lock);
  
  	return signalled;
  }
  
  static void vmpressure_work_fn(struct work_struct *work)
  {
  	struct vmpressure *vmpr = work_to_vmpressure(work);
  	unsigned long scanned;
  	unsigned long reclaimed;
  
  	spin_lock(&vmpr->sr_lock);
  	/*
  	 * Several contexts might be calling vmpressure(), so it is
  	 * possible that the work was rescheduled again before the old
  	 * work context cleared the counters. In that case we will run
  	 * just after the old work returns, but then scanned might be zero
  	 * here. No need for any locks here since we don't care if
  	 * vmpr->reclaimed is in sync.
  	 */
  	scanned = vmpr->scanned;
  	if (!scanned) {
  		spin_unlock(&vmpr->sr_lock);
  		return;
  	}
  
  	reclaimed = vmpr->reclaimed;
  	vmpr->scanned = 0;
  	vmpr->reclaimed = 0;
  	spin_unlock(&vmpr->sr_lock);
  
  	do {
  		if (vmpressure_event(vmpr, scanned, reclaimed))
  			break;
  		/*
  		 * If not handled, propagate the event upward into the
  		 * hierarchy.
  		 */
  	} while ((vmpr = vmpressure_parent(vmpr)));
  }
  
  /**
   * vmpressure() - Account memory pressure through scanned/reclaimed ratio
   * @gfp:	reclaimer's gfp mask
   * @memcg:	cgroup memory controller handle
   * @scanned:	number of pages scanned
   * @reclaimed:	number of pages reclaimed
   *
   * This function should be called from the vmscan reclaim path to account
   * "instantaneous" memory pressure (scanned/reclaimed ratio). The raw
   * pressure index is then further refined and averaged over time.
   *
   * This function does not return any value.
   */
  void vmpressure(gfp_t gfp, struct mem_cgroup *memcg,
  		unsigned long scanned, unsigned long reclaimed)
  {
  	struct vmpressure *vmpr = memcg_to_vmpressure(memcg);
  
  	/*
  	 * Here we only want to account pressure that userland is able to
  	 * help us with. For example, suppose that DMA zone is under
  	 * pressure; if we notify userland about that kind of pressure,
  	 * then it will be mostly a waste as it will trigger unnecessary
  	 * freeing of memory by userland (since userland is more likely to
  	 * have HIGHMEM/MOVABLE pages instead of the DMA fallback). That
  	 * is why we include only movable, highmem and FS/IO pages.
  	 * Indirect reclaim (kswapd) sets sc->gfp_mask to GFP_KERNEL, so
  	 * we account it too.
  	 */
  	if (!(gfp & (__GFP_HIGHMEM | __GFP_MOVABLE | __GFP_IO | __GFP_FS)))
  		return;
  
  	/*
  	 * If we got here with no pages scanned, then that is an indicator
  	 * that reclaimer was unable to find any shrinkable LRUs at the
  	 * current scanning depth. But it does not mean that we should
  	 * report the critical pressure, yet. If the scanning priority
  	 * (scanning depth) goes too high (deep), we will be notified
  	 * through vmpressure_prio(). But so far, keep calm.
  	 */
  	if (!scanned)
  		return;
  
  	spin_lock(&vmpr->sr_lock);
  	vmpr->scanned += scanned;
  	vmpr->reclaimed += reclaimed;
  	scanned = vmpr->scanned;
  	spin_unlock(&vmpr->sr_lock);
  
  	if (scanned < vmpressure_win)
  		return;
  	schedule_work(&vmpr->work);
  }
  
  /**
   * vmpressure_prio() - Account memory pressure through reclaimer priority level
   * @gfp:	reclaimer's gfp mask
   * @memcg:	cgroup memory controller handle
   * @prio:	reclaimer's priority
   *
   * This function should be called from the reclaim path every time when
   * the vmscan's reclaiming priority (scanning depth) changes.
   *
   * This function does not return any value.
   */
  void vmpressure_prio(gfp_t gfp, struct mem_cgroup *memcg, int prio)
  {
  	/*
  	 * We only use prio for accounting critical level. For more info
  	 * see comment for vmpressure_level_critical_prio variable above.
  	 */
  	if (prio > vmpressure_level_critical_prio)
  		return;
  
  	/*
  	 * OK, the prio is below the threshold, updating vmpressure
  	 * information before shrinker dives into long shrinking of long
  	 * range vmscan. Passing scanned = vmpressure_win, reclaimed = 0
  	 * to the vmpressure() basically means that we signal 'critical'
  	 * level.
  	 */
  	vmpressure(gfp, memcg, vmpressure_win, 0);
  }
  
  /**
   * vmpressure_register_event() - Bind vmpressure notifications to an eventfd
   * @memcg:	memcg that is interested in vmpressure notifications
   * @eventfd:	eventfd context to link notifications with
   * @args:	event arguments (used to set up a pressure level threshold)
   *
   * This function associates eventfd context with the vmpressure
   * infrastructure, so that the notifications will be delivered to the
   * @eventfd. The @args parameter is a string that denotes pressure level
   * threshold (one of vmpressure_str_levels, i.e. "low", "medium", or
   * "critical").
   *
   * To be used as memcg event method.
   */
  int vmpressure_register_event(struct mem_cgroup *memcg,
  			      struct eventfd_ctx *eventfd, const char *args)
  {
  	struct vmpressure *vmpr = memcg_to_vmpressure(memcg);
  	struct vmpressure_event *ev;
  	int level;
  
  	for (level = 0; level < VMPRESSURE_NUM_LEVELS; level++) {
  		if (!strcmp(vmpressure_str_levels[level], args))
  			break;
  	}
  
  	if (level >= VMPRESSURE_NUM_LEVELS)
  		return -EINVAL;
  
  	ev = kzalloc(sizeof(*ev), GFP_KERNEL);
  	if (!ev)
  		return -ENOMEM;
  
  	ev->efd = eventfd;
  	ev->level = level;
  
  	mutex_lock(&vmpr->events_lock);
  	list_add(&ev->node, &vmpr->events);
  	mutex_unlock(&vmpr->events_lock);
  
  	return 0;
  }
  
  /**
   * vmpressure_unregister_event() - Unbind eventfd from vmpressure
   * @memcg:	memcg handle
   * @eventfd:	eventfd context that was used to link vmpressure with the @cg
   *
   * This function does internal manipulations to detach the @eventfd from
   * the vmpressure notifications, and then frees internal resources
   * associated with the @eventfd (but the @eventfd itself is not freed).
   *
   * To be used as memcg event method.
   */
  void vmpressure_unregister_event(struct mem_cgroup *memcg,
  				 struct eventfd_ctx *eventfd)
  {
  	struct vmpressure *vmpr = memcg_to_vmpressure(memcg);
  	struct vmpressure_event *ev;
  
  	mutex_lock(&vmpr->events_lock);
  	list_for_each_entry(ev, &vmpr->events, node) {
  		if (ev->efd != eventfd)
  			continue;
  		list_del(&ev->node);
  		kfree(ev);
  		break;
  	}
  	mutex_unlock(&vmpr->events_lock);
  }
  
  /**
   * vmpressure_init() - Initialize vmpressure control structure
   * @vmpr:	Structure to be initialized
   *
   * This function should be called on every allocated vmpressure structure
   * before any usage.
   */
  void vmpressure_init(struct vmpressure *vmpr)
  {
  	spin_lock_init(&vmpr->sr_lock);
  	mutex_init(&vmpr->events_lock);
  	INIT_LIST_HEAD(&vmpr->events);
  	INIT_WORK(&vmpr->work, vmpressure_work_fn);
  }
  
  /**
   * vmpressure_cleanup() - shuts down vmpressure control structure
   * @vmpr:	Structure to be cleaned up
   *
   * This function should be called before the structure in which it is
   * embedded is cleaned up.
   */
  void vmpressure_cleanup(struct vmpressure *vmpr)
  {
  	/*
  	 * Make sure there is no pending work before eventfd infrastructure
  	 * goes away.
  	 */
  	flush_work(&vmpr->work);
  }