1 files changed, 384 insertions, 0 deletions
diff --git a/lib/proportions.c b/lib/proportions.c
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+/*
+ * Floating proportions
+ *
+ *  Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
+ *
+ * Description:
+ *
+ * The floating proportion is a time derivative with an exponentially decaying
+ * history:
+ *
+ *   p_{j} = \Sum_{i=0} (dx_{j}/dt_{-i}) / 2^(1+i)
+ *
+ * Where j is an element from {prop_local}, x_{j} is j's number of events,
+ * and i the time period over which the differential is taken. So d/dt_{-i} is
+ * the differential over the i-th last period.
+ *
+ * The decaying history gives smooth transitions. The time differential carries
+ * the notion of speed.
+ *
+ * The denominator is 2^(1+i) because we want the series to be normalised, ie.
+ *
+ *   \Sum_{i=0} 1/2^(1+i) = 1
+ *
+ * Further more, if we measure time (t) in the same events as x; so that:
+ *
+ *   t = \Sum_{j} x_{j}
+ *
+ * we get that:
+ *
+ *   \Sum_{j} p_{j} = 1
+ *
+ * Writing this in an iterative fashion we get (dropping the 'd's):
+ *
+ *   if (++x_{j}, ++t > period)
+ *     t /= 2;
+ *     for_each (j)
+ *       x_{j} /= 2;
+ *
+ * so that:
+ *
+ *   p_{j} = x_{j} / t;
+ *
+ * We optimize away the '/= 2' for the global time delta by noting that:
+ *
+ *   if (++t > period) t /= 2:
+ *
+ * Can be approximated by:
+ *
+ *   period/2 + (++t % period/2)
+ *
+ * [ Furthermore, when we choose period to be 2^n it can be written in terms of
+ *   binary operations and wraparound artefacts disappear. ]
+ *
+ * Also note that this yields a natural counter of the elapsed periods:
+ *
+ *   c = t / (period/2)
+ *
+ * [ Its monotonic increasing property can be applied to mitigate the wrap-
+ *   around issue. ]
+ *
+ * This allows us to do away with the loop over all prop_locals on each period
+ * expiration. By remembering the period count under which it was last accessed
+ * as c_{j}, we can obtain the number of 'missed' cycles from:
+ *
+ *   c - c_{j}
+ *
+ * We can then lazily catch up to the global period count every time we are
+ * going to use x_{j}, by doing:
+ *
+ *   x_{j} /= 2^(c - c_{j}), c_{j} = c
+ */
+
+#include <linux/proportions.h>
+#include <linux/rcupdate.h>
+
+/*
+ * Limit the time part in order to ensure there are some bits left for the
+ * cycle counter.
+ */
+#define PROP_MAX_SHIFT (3*BITS_PER_LONG/4)
+
+int prop_descriptor_init(struct prop_descriptor *pd, int shift)
+{
+	int err;
+
+	if (shift > PROP_MAX_SHIFT)
+		shift = PROP_MAX_SHIFT;
+
+	pd->index = 0;
+	pd->pg[0].shift = shift;
+	mutex_init(&pd->mutex);
+	err = percpu_counter_init_irq(&pd->pg[0].events, 0);
+	if (err)
+		goto out;
+
+	err = percpu_counter_init_irq(&pd->pg[1].events, 0);
+	if (err)
+		percpu_counter_destroy(&pd->pg[0].events);
+
+out:
+	return err;
+}
+
+/*
+ * We have two copies, and flip between them to make it seem like an atomic
+ * update. The update is not really atomic wrt the events counter, but
+ * it is internally consistent with the bit layout depending on shift.
+ *
+ * We copy the events count, move the bits around and flip the index.
+ */
+void prop_change_shift(struct prop_descriptor *pd, int shift)
+{
+	int index;
+	int offset;
+	u64 events;
+	unsigned long flags;
+
+	if (shift > PROP_MAX_SHIFT)
+		shift = PROP_MAX_SHIFT;
+
+	mutex_lock(&pd->mutex);
+
+	index = pd->index ^ 1;
+	offset = pd->pg[pd->index].shift - shift;
+	if (!offset)
+		goto out;
+
+	pd->pg[index].shift = shift;
+
+	local_irq_save(flags);
+	events = percpu_counter_sum(&pd->pg[pd->index].events);
+	if (offset < 0)
+		events <<= -offset;
+	else
+		events >>= offset;
+	percpu_counter_set(&pd->pg[index].events, events);
+
+	/*
+	 * ensure the new pg is fully written before the switch
+	 */
+	smp_wmb();
+	pd->index = index;
+	local_irq_restore(flags);
+
+	synchronize_rcu();
+
+out:
+	mutex_unlock(&pd->mutex);
+}
+
+/*
+ * wrap the access to the data in an rcu_read_lock() section;
+ * this is used to track the active references.
+ */
+static struct prop_global *prop_get_global(struct prop_descriptor *pd)
+{
+	int index;
+
+	rcu_read_lock();
+	index = pd->index;
+	/*
+	 * match the wmb from vcd_flip()
+	 */
+	smp_rmb();
+	return &pd->pg[index];
+}
+
+static void prop_put_global(struct prop_descriptor *pd, struct prop_global *pg)
+{
+	rcu_read_unlock();
+}
+
+static void
+prop_adjust_shift(int *pl_shift, unsigned long *pl_period, int new_shift)
+{
+	int offset = *pl_shift - new_shift;
+
+	if (!offset)
+		return;
+
+	if (offset < 0)
+		*pl_period <<= -offset;
+	else
+		*pl_period >>= offset;
+
+	*pl_shift = new_shift;
+}
+
+/*
+ * PERCPU
+ */
+
+int prop_local_init_percpu(struct prop_local_percpu *pl)
+{
+	spin_lock_init(&pl->lock);
+	pl->shift = 0;
+	pl->period = 0;
+	return percpu_counter_init_irq(&pl->events, 0);
+}
+
+void prop_local_destroy_percpu(struct prop_local_percpu *pl)
+{
+	percpu_counter_destroy(&pl->events);
+}
+
+/*
+ * Catch up with missed period expirations.
+ *
+ *   until (c_{j} == c)
+ *     x_{j} -= x_{j}/2;
+ *     c_{j}++;
+ */
+static
+void prop_norm_percpu(struct prop_global *pg, struct prop_local_percpu *pl)
+{
+	unsigned long period = 1UL << (pg->shift - 1);
+	unsigned long period_mask = ~(period - 1);
+	unsigned long global_period;
+	unsigned long flags;
+
+	global_period = percpu_counter_read(&pg->events);
+	global_period &= period_mask;
+
+	/*
+	 * Fast path - check if the local and global period count still match
+	 * outside of the lock.
+	 */
+	if (pl->period == global_period)
+		return;
+
+	spin_lock_irqsave(&pl->lock, flags);
+	prop_adjust_shift(&pl->shift, &pl->period, pg->shift);
+	/*
+	 * For each missed period, we half the local counter.
+	 * basically:
+	 *   pl->events >> (global_period - pl->period);
+	 *
+	 * but since the distributed nature of percpu counters make division
+	 * rather hard, use a regular subtraction loop. This is safe, because
+	 * the events will only every be incremented, hence the subtraction
+	 * can never result in a negative number.
+	 */
+	while (pl->period != global_period) {
+		unsigned long val = percpu_counter_read(&pl->events);
+		unsigned long half = (val + 1) >> 1;
+
+		/*
+		 * Half of zero won't be much less, break out.
+		 * This limits the loop to shift iterations, even
+		 * if we missed a million.
+		 */
+		if (!val)
+			break;
+
+		percpu_counter_add(&pl->events, -half);
+		pl->period += period;
+	}
+	pl->period = global_period;
+	spin_unlock_irqrestore(&pl->lock, flags);
+}
+
+/*
+ *   ++x_{j}, ++t
+ */
+void __prop_inc_percpu(struct prop_descriptor *pd, struct prop_local_percpu *pl)
+{
+	struct prop_global *pg = prop_get_global(pd);
+
+	prop_norm_percpu(pg, pl);
+	percpu_counter_add(&pl->events, 1);
+	percpu_counter_add(&pg->events, 1);
+	prop_put_global(pd, pg);
+}
+
+/*
+ * Obtain a fraction of this proportion
+ *
+ *   p_{j} = x_{j} / (period/2 + t % period/2)
+ */
+void prop_fraction_percpu(struct prop_descriptor *pd,
+		struct prop_local_percpu *pl,
+		long *numerator, long *denominator)
+{
+	struct prop_global *pg = prop_get_global(pd);
+	unsigned long period_2 = 1UL << (pg->shift - 1);
+	unsigned long counter_mask = period_2 - 1;
+	unsigned long global_count;
+
+	prop_norm_percpu(pg, pl);
+	*numerator = percpu_counter_read_positive(&pl->events);
+
+	global_count = percpu_counter_read(&pg->events);
+	*denominator = period_2 + (global_count & counter_mask);
+
+	prop_put_global(pd, pg);
+}
+
+/*
+ * SINGLE
+ */
+
+int prop_local_init_single(struct prop_local_single *pl)
+{
+	spin_lock_init(&pl->lock);
+	pl->shift = 0;
+	pl->period = 0;
+	pl->events = 0;
+	return 0;
+}
+
+void prop_local_destroy_single(struct prop_local_single *pl)
+{
+}
+
+/*
+ * Catch up with missed period expirations.
+ */
+static
+void prop_norm_single(struct prop_global *pg, struct prop_local_single *pl)
+{
+	unsigned long period = 1UL << (pg->shift - 1);
+	unsigned long period_mask = ~(period - 1);
+	unsigned long global_period;
+	unsigned long flags;
+
+	global_period = percpu_counter_read(&pg->events);
+	global_period &= period_mask;
+
+	/*
+	 * Fast path - check if the local and global period count still match
+	 * outside of the lock.
+	 */
+	if (pl->period == global_period)
+		return;
+
+	spin_lock_irqsave(&pl->lock, flags);
+	prop_adjust_shift(&pl->shift, &pl->period, pg->shift);
+	/*
+	 * For each missed period, we half the local counter.
+	 */
+	period = (global_period - pl->period) >> (pg->shift - 1);
+	if (likely(period < BITS_PER_LONG))
+		pl->events >>= period;
+	else
+		pl->events = 0;
+	pl->period = global_period;
+	spin_unlock_irqrestore(&pl->lock, flags);
+}
+
+/*
+ *   ++x_{j}, ++t
+ */
+void __prop_inc_single(struct prop_descriptor *pd, struct prop_local_single *pl)
+{
+	struct prop_global *pg = prop_get_global(pd);
+
+	prop_norm_single(pg, pl);
+	pl->events++;
+	percpu_counter_add(&pg->events, 1);
+	prop_put_global(pd, pg);
+}
+
+/*
+ * Obtain a fraction of this proportion
+ *
+ *   p_{j} = x_{j} / (period/2 + t % period/2)
+ */
+void prop_fraction_single(struct prop_descriptor *pd,
+	       	struct prop_local_single *pl,
+		long *numerator, long *denominator)
+{
+	struct prop_global *pg = prop_get_global(pd);
+	unsigned long period_2 = 1UL << (pg->shift - 1);
+	unsigned long counter_mask = period_2 - 1;
+	unsigned long global_count;
+
+	prop_norm_single(pg, pl);
+	*numerator = pl->events;
+
+	global_count = percpu_counter_read(&pg->events);
+	*denominator = period_2 + (global_count & counter_mask);
+
+	prop_put_global(pd, pg);
+}