Merge tag 'writeback-proportions' of git://git.kernel.org/pub/scm/linux/kernel/git/wfg/linux

Pull writeback updates from Wu Fengguang:
 "Use time based periods to age the writeback proportions, which can
  adapt equally well to fast/slow devices."

Fix up trivial conflict in comment in fs/sync.c

* tag 'writeback-proportions' of git://git.kernel.org/pub/scm/linux/kernel/git/wfg/linux:
  writeback: Fix some comment errors
  block: Convert BDI proportion calculations to flexible proportions
  lib: Fix possible deadlock in flexible proportion code
  lib: Proportions with flexible period

1 files changed, 272 insertions, 0 deletions

diff --git a/lib/flex_proportions.c b/lib/flex_proportions.c
new file mode 100644
index 000000000000..c785554f9523
--- /dev/null
+++ b/lib/flex_proportions.c
@@ -0,0 +1,272 @@
+/*
+ *  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)
+{
+	int err;
+
+	p->period = 0;
+	/* Use 1 to avoid dealing with periods with 0 events... */
+	err = percpu_counter_init(&p->events, 1);
+	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)
+{
+	u64 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)
+{
+	int err;
+
+	err = percpu_counter_init(&pl->events, 0);
+	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);
+}