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