diff options
Diffstat (limited to 'lib')
| -rw-r--r-- | lib/Makefile | 3 | ||||
| -rw-r--r-- | lib/proportions.c | 384 |
2 files changed, 386 insertions, 1 deletions
diff --git a/lib/Makefile b/lib/Makefile index 6c4ea33bb2cb..c5f215d509d3 100644 --- a/lib/Makefile +++ b/lib/Makefile @@ -5,7 +5,8 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \ rbtree.o radix-tree.o dump_stack.o \ idr.o int_sqrt.o bitmap.o extable.o prio_tree.o \ - sha1.o irq_regs.o reciprocal_div.o argv_split.o + sha1.o irq_regs.o reciprocal_div.o argv_split.o \ + proportions.o lib-$(CONFIG_MMU) += ioremap.o lib-$(CONFIG_SMP) += cpumask.o diff --git a/lib/proportions.c b/lib/proportions.c new file mode 100644 index 000000000000..332d8c58184d --- /dev/null +++ b/lib/proportions.c @@ -0,0 +1,384 @@ +/* + * 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); +} |