percpu_counter.c

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/*
 * Fast batching percpu counters.
 */

#include <linux/percpu_counter.h>
#include <linux/notifier.h>
#include <linux/mutex.h>
#include <linux/init.h>
#include <linux/cpu.h>
#include <linux/module.h>
#include <linux/debugobjects.h>

#ifdef CONFIG_HOTPLUG_CPU
static LIST_HEAD(percpu_counters);
static DEFINE_SPINLOCK(percpu_counters_lock);
#endif

#ifdef CONFIG_DEBUG_OBJECTS_PERCPU_COUNTER

static struct debug_obj_descr percpu_counter_debug_descr;

static int percpu_counter_fixup_free(void *addr, enum debug_obj_state state)
{
    struct percpu_counter *fbc = addr;

    switch (state) {
    case ODEBUG_STATE_ACTIVE:
        percpu_counter_destroy(fbc);
        debug_object_free(fbc, &percpu_counter_debug_descr);
        return 1;
    default:
        return 0;
    }
}

static struct debug_obj_descr percpu_counter_debug_descr = {
    .name       = "percpu_counter",
    .fixup_free = percpu_counter_fixup_free,
};

static inline void debug_percpu_counter_activate(struct percpu_counter *fbc)
{
    debug_object_init(fbc, &percpu_counter_debug_descr);
    debug_object_activate(fbc, &percpu_counter_debug_descr);
}

static inline void debug_percpu_counter_deactivate(struct percpu_counter *fbc)
{
    debug_object_deactivate(fbc, &percpu_counter_debug_descr);
    debug_object_free(fbc, &percpu_counter_debug_descr);
}

#else   /* CONFIG_DEBUG_OBJECTS_PERCPU_COUNTER */
static inline void debug_percpu_counter_activate(struct percpu_counter *fbc)
{ }
static inline void debug_percpu_counter_deactivate(struct percpu_counter *fbc)
{ }
#endif  /* CONFIG_DEBUG_OBJECTS_PERCPU_COUNTER */

void percpu_counter_set(struct percpu_counter *fbc, s64 amount)
{
    int cpu;

    raw_spin_lock(&fbc->lock);
    for_each_possible_cpu(cpu) {
        s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
        *pcount = 0;
    }
    fbc->count = amount;
    raw_spin_unlock(&fbc->lock);
}
EXPORT_SYMBOL(percpu_counter_set);

void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch)
{
    s64 count;

    preempt_disable();
    count = __this_cpu_read(*fbc->counters) + amount;
    if (count >= batch || count <= -batch) {
        raw_spin_lock(&fbc->lock);
        fbc->count += count;
        __this_cpu_write(*fbc->counters, 0);
        raw_spin_unlock(&fbc->lock);
    } else {
        __this_cpu_write(*fbc->counters, count);
    }
    preempt_enable();
}
EXPORT_SYMBOL(__percpu_counter_add);

/*
 * Add up all the per-cpu counts, return the result.  This is a more accurate
 * but much slower version of percpu_counter_read_positive()
 */
s64 __percpu_counter_sum(struct percpu_counter *fbc)
{
    s64 ret;
    int cpu;

    raw_spin_lock(&fbc->lock);
    ret = fbc->count;
    for_each_online_cpu(cpu) {
        s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
        ret += *pcount;
    }
    raw_spin_unlock(&fbc->lock);
    return ret;
}
EXPORT_SYMBOL(__percpu_counter_sum);

int __percpu_counter_init(struct percpu_counter *fbc, s64 amount,
              struct lock_class_key *key)
{
    raw_spin_lock_init(&fbc->lock);
    lockdep_set_class(&fbc->lock, key);
    fbc->count = amount;
    fbc->counters = alloc_percpu(s32);
    if (!fbc->counters)
        return -ENOMEM;

    debug_percpu_counter_activate(fbc);

#ifdef CONFIG_HOTPLUG_CPU
    INIT_LIST_HEAD(&fbc->list);
    spin_lock(&percpu_counters_lock);
    list_add(&fbc->list, &percpu_counters);
    spin_unlock(&percpu_counters_lock);
#endif
    return 0;
}
EXPORT_SYMBOL(__percpu_counter_init);

void percpu_counter_destroy(struct percpu_counter *fbc)
{
    if (!fbc->counters)
        return;

    debug_percpu_counter_deactivate(fbc);

#ifdef CONFIG_HOTPLUG_CPU
    spin_lock(&percpu_counters_lock);
    list_del(&fbc->list);
    spin_unlock(&percpu_counters_lock);
#endif
    free_percpu(fbc->counters);
    fbc->counters = NULL;
}
EXPORT_SYMBOL(percpu_counter_destroy);

int percpu_counter_batch __read_mostly = 32;
EXPORT_SYMBOL(percpu_counter_batch);

static void compute_batch_value(void)
{
    int nr = num_online_cpus();

    percpu_counter_batch = max(32, nr*2);
}

static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
                    unsigned long action, void *hcpu)
{
#ifdef CONFIG_HOTPLUG_CPU
    unsigned int cpu;
    struct percpu_counter *fbc;

    compute_batch_value();
    if (action != CPU_DEAD)
        return NOTIFY_OK;

    cpu = (unsigned long)hcpu;
    spin_lock(&percpu_counters_lock);
    list_for_each_entry(fbc, &percpu_counters, list) {
        s32 *pcount;
        unsigned long flags;

        raw_spin_lock_irqsave(&fbc->lock, flags);
        pcount = per_cpu_ptr(fbc->counters, cpu);
        fbc->count += *pcount;
        *pcount = 0;
        raw_spin_unlock_irqrestore(&fbc->lock, flags);
    }
    spin_unlock(&percpu_counters_lock);
#endif
    return NOTIFY_OK;
}

/*
 * Compare counter against given value.
 * Return 1 if greater, 0 if equal and -1 if less
 */
int percpu_counter_compare(struct percpu_counter *fbc, s64 rhs)
{
    s64 count;

    count = percpu_counter_read(fbc);
    /* Check to see if rough count will be sufficient for comparison */
    if (abs(count - rhs) > (percpu_counter_batch*num_online_cpus())) {
        if (count > rhs)
            return 1;
        else
            return -1;
    }
    /* Need to use precise count */
    count = percpu_counter_sum(fbc);
    if (count > rhs)
        return 1;
    else if (count < rhs)
        return -1;
    else
        return 0;
}
EXPORT_SYMBOL(percpu_counter_compare);

static int __init percpu_counter_startup(void)
{
    compute_batch_value();
    hotcpu_notifier(percpu_counter_hotcpu_callback, 0);
    return 0;
}
module_init(percpu_counter_startup);