/*
   *  Floating proportions with flexible aging period
   *
   *   Copyright (C) 2011, SUSE, Jan Kara <jack@suse.cz>
   *
   * The goal of this code is: Given different types of event, measure proportion
   * of each type of event over time. The proportions are measured with
   * exponentially decaying history to give smooth transitions. A formula
   * expressing proportion of event of type 'j' is:
   *
   *   p_{j} = (\Sum_{i>=0} x_{i,j}/2^{i+1})/(\Sum_{i>=0} x_i/2^{i+1})
   *
   * Where x_{i,j} is j's number of events in i-th last time period and x_i is
   * total number of events in i-th last time period.
   *
   * Note that p_{j}'s are normalised, i.e.
   *
   *   \Sum_{j} p_{j} = 1,
   *
   * This formula can be straightforwardly computed by maintaing denominator
   * (let's call it 'd') and for each event type its numerator (let's call it
   * 'n_j'). When an event of type 'j' happens, we simply need to do:
   *   n_j++; d++;
   *
   * When a new period is declared, we could do:
   *   d /= 2
   *   for each j
   *     n_j /= 2
   *
   * To avoid iteration over all event types, we instead shift numerator of event
   * j lazily when someone asks for a proportion of event j or when event j
   * occurs. This can bit trivially implemented by remembering last period in
   * which something happened with proportion of type j.
   */
  #include <linux/flex_proportions.h>
  
  int fprop_global_init(struct fprop_global *p, gfp_t gfp)
  {
  	int err;
  
  	p->period = 0;
  	/* Use 1 to avoid dealing with periods with 0 events... */
  	err = percpu_counter_init(&p->events, 1, gfp);
  	if (err)
  		return err;
  	seqcount_init(&p->sequence);
  	return 0;
  }
  
  void fprop_global_destroy(struct fprop_global *p)
  {
  	percpu_counter_destroy(&p->events);
  }
  
  /*
   * Declare @periods new periods. It is upto the caller to make sure period
   * transitions cannot happen in parallel.
   *
   * The function returns true if the proportions are still defined and false
   * if aging zeroed out all events. This can be used to detect whether declaring
   * further periods has any effect.
   */
  bool fprop_new_period(struct fprop_global *p, int periods)
  {
  	s64 events;
  	unsigned long flags;
  
  	local_irq_save(flags);
  	events = percpu_counter_sum(&p->events);
  	/*
  	 * Don't do anything if there are no events.
  	 */
  	if (events <= 1) {
  		local_irq_restore(flags);
  		return false;
  	}
  	write_seqcount_begin(&p->sequence);
  	if (periods < 64)
  		events -= events >> periods;
  	/* Use addition to avoid losing events happening between sum and set */
  	percpu_counter_add(&p->events, -events);
  	p->period += periods;
  	write_seqcount_end(&p->sequence);
  	local_irq_restore(flags);
  
  	return true;
  }
  
  /*
   * ---- SINGLE ----
   */
  
  int fprop_local_init_single(struct fprop_local_single *pl)
  {
  	pl->events = 0;
  	pl->period = 0;
  	raw_spin_lock_init(&pl->lock);
  	return 0;
  }
  
  void fprop_local_destroy_single(struct fprop_local_single *pl)
  {
  }
  
  static void fprop_reflect_period_single(struct fprop_global *p,
  					struct fprop_local_single *pl)
  {
  	unsigned int period = p->period;
  	unsigned long flags;
  
  	/* Fast path - period didn't change */
  	if (pl->period == period)
  		return;
  	raw_spin_lock_irqsave(&pl->lock, flags);
  	/* Someone updated pl->period while we were spinning? */
  	if (pl->period >= period) {
  		raw_spin_unlock_irqrestore(&pl->lock, flags);
  		return;
  	}
  	/* Aging zeroed our fraction? */
  	if (period - pl->period < BITS_PER_LONG)
  		pl->events >>= period - pl->period;
  	else
  		pl->events = 0;
  	pl->period = period;
  	raw_spin_unlock_irqrestore(&pl->lock, flags);
  }
  
  /* Event of type pl happened */
  void __fprop_inc_single(struct fprop_global *p, struct fprop_local_single *pl)
  {
  	fprop_reflect_period_single(p, pl);
  	pl->events++;
  	percpu_counter_add(&p->events, 1);
  }
  
  /* Return fraction of events of type pl */
  void fprop_fraction_single(struct fprop_global *p,
  			   struct fprop_local_single *pl,
  			   unsigned long *numerator, unsigned long *denominator)
  {
  	unsigned int seq;
  	s64 num, den;
  
  	do {
  		seq = read_seqcount_begin(&p->sequence);
  		fprop_reflect_period_single(p, pl);
  		num = pl->events;
  		den = percpu_counter_read_positive(&p->events);
  	} while (read_seqcount_retry(&p->sequence, seq));
  
  	/*
  	 * Make fraction <= 1 and denominator > 0 even in presence of percpu
  	 * counter errors
  	 */
  	if (den <= num) {
  		if (num)
  			den = num;
  		else
  			den = 1;
  	}
  	*denominator = den;
  	*numerator = num;
  }
  
  /*
   * ---- PERCPU ----
   */
  #define PROP_BATCH (8*(1+ilog2(nr_cpu_ids)))
  
  int fprop_local_init_percpu(struct fprop_local_percpu *pl, gfp_t gfp)
  {
  	int err;
  
  	err = percpu_counter_init(&pl->events, 0, gfp);
  	if (err)
  		return err;
  	pl->period = 0;
  	raw_spin_lock_init(&pl->lock);
  	return 0;
  }
  
  void fprop_local_destroy_percpu(struct fprop_local_percpu *pl)
  {
  	percpu_counter_destroy(&pl->events);
  }
  
  static void fprop_reflect_period_percpu(struct fprop_global *p,
  					struct fprop_local_percpu *pl)
  {
  	unsigned int period = p->period;
  	unsigned long flags;
  
  	/* Fast path - period didn't change */
  	if (pl->period == period)
  		return;
  	raw_spin_lock_irqsave(&pl->lock, flags);
  	/* Someone updated pl->period while we were spinning? */
  	if (pl->period >= period) {
  		raw_spin_unlock_irqrestore(&pl->lock, flags);
  		return;
  	}
  	/* Aging zeroed our fraction? */
  	if (period - pl->period < BITS_PER_LONG) {
  		s64 val = percpu_counter_read(&pl->events);
  
  		if (val < (nr_cpu_ids * PROP_BATCH))
  			val = percpu_counter_sum(&pl->events);
  
  		__percpu_counter_add(&pl->events,
  			-val + (val >> (period-pl->period)), PROP_BATCH);
  	} else
  		percpu_counter_set(&pl->events, 0);
  	pl->period = period;
  	raw_spin_unlock_irqrestore(&pl->lock, flags);
  }
  
  /* Event of type pl happened */
  void __fprop_inc_percpu(struct fprop_global *p, struct fprop_local_percpu *pl)
  {
  	fprop_reflect_period_percpu(p, pl);
  	__percpu_counter_add(&pl->events, 1, PROP_BATCH);
  	percpu_counter_add(&p->events, 1);
  }
  
  void fprop_fraction_percpu(struct fprop_global *p,
  			   struct fprop_local_percpu *pl,
  			   unsigned long *numerator, unsigned long *denominator)
  {
  	unsigned int seq;
  	s64 num, den;
  
  	do {
  		seq = read_seqcount_begin(&p->sequence);
  		fprop_reflect_period_percpu(p, pl);
  		num = percpu_counter_read_positive(&pl->events);
  		den = percpu_counter_read_positive(&p->events);
  	} while (read_seqcount_retry(&p->sequence, seq));
  
  	/*
  	 * Make fraction <= 1 and denominator > 0 even in presence of percpu
  	 * counter errors
  	 */
  	if (den <= num) {
  		if (num)
  			den = num;
  		else
  			den = 1;
  	}
  	*denominator = den;
  	*numerator = num;
  }
  
  /*
   * Like __fprop_inc_percpu() except that event is counted only if the given
   * type has fraction smaller than @max_frac/FPROP_FRAC_BASE
   */
  void __fprop_inc_percpu_max(struct fprop_global *p,
  			    struct fprop_local_percpu *pl, int max_frac)
  {
  	if (unlikely(max_frac < FPROP_FRAC_BASE)) {
  		unsigned long numerator, denominator;
  
  		fprop_fraction_percpu(p, pl, &numerator, &denominator);
  		if (numerator >
  		    (((u64)denominator) * max_frac) >> FPROP_FRAC_SHIFT)
  			return;
  	} else
  		fprop_reflect_period_percpu(p, pl);
  	__percpu_counter_add(&pl->events, 1, PROP_BATCH);
  	percpu_counter_add(&p->events, 1);
  }