/*
 * Percpu IDA library
 *
 * Copyright (C) 2013 Datera, Inc. Kent Overstreet
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2, or (at
 * your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 */

#include <linux/bitmap.h>
#include <linux/bitops.h>
#include <linux/bug.h>
#include <linux/err.h>
#include <linux/export.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/percpu.h>
#include <linux/sched.h>
#include <linux/string.h>
#include <linux/spinlock.h>
#include <linux/percpu_ida.h>

struct percpu_ida_cpu {
        /*
         * Even though this is percpu, we need a lock for tag stealing by remote
         * CPUs:
         */
        spinlock_t                      lock;

        /* nr_free/freelist form a stack of free IDs */
        unsigned                        nr_free;
        unsigned                        freelist[];
};

static inline void move_tags(unsigned *dst, unsigned *dst_nr,
                             unsigned *src, unsigned *src_nr,
                             unsigned nr)
{
        *src_nr -= nr;
        memcpy(dst + *dst_nr, src + *src_nr, sizeof(unsigned) * nr);
        *dst_nr += nr;
}

/*
 * Try to steal tags from a remote cpu's percpu freelist.
 *
 * We first check how many percpu freelists have tags
 *
 * Then we iterate through the cpus until we find some tags - we don't attempt
 * to find the "best" cpu to steal from, to keep cacheline bouncing to a
 * minimum.
 */
static inline void steal_tags(struct percpu_ida *pool,
                              struct percpu_ida_cpu *tags)
{
        unsigned cpus_have_tags, cpu = pool->cpu_last_stolen;
        struct percpu_ida_cpu *remote;

        for (cpus_have_tags = cpumask_weight(&pool->cpus_have_tags);
             cpus_have_tags; cpus_have_tags--) {
                cpu = cpumask_next(cpu, &pool->cpus_have_tags);

                if (cpu >= nr_cpu_ids) {
                        cpu = cpumask_first(&pool->cpus_have_tags);
                        if (cpu >= nr_cpu_ids)
                                BUG();
                }

                pool->cpu_last_stolen = cpu;
                remote = per_cpu_ptr(pool->tag_cpu, cpu);

                cpumask_clear_cpu(cpu, &pool->cpus_have_tags);

                if (remote == tags)
                        continue;

                spin_lock(&remote->lock);

                if (remote->nr_free) {
                        memcpy(tags->freelist,
                               remote->freelist,
                               sizeof(unsigned) * remote->nr_free);

                        tags->nr_free = remote->nr_free;
                        remote->nr_free = 0;
                }

                spin_unlock(&remote->lock);

                if (tags->nr_free)
                        break;
        }
}

/*
 * Pop up to IDA_PCPU_BATCH_MOVE IDs off the global freelist, and push them onto
 * our percpu freelist:
 */
static inline void alloc_global_tags(struct percpu_ida *pool,
                                     struct percpu_ida_cpu *tags)
{
        move_tags(tags->freelist, &tags->nr_free,
                  pool->freelist, &pool->nr_free,
                  min(pool->nr_free, pool->percpu_batch_size));
}

static inline unsigned alloc_local_tag(struct percpu_ida_cpu *tags)
{
        int tag = -ENOSPC;

        spin_lock(&tags->lock);
        if (tags->nr_free)
                tag = tags->freelist[--tags->nr_free];
        spin_unlock(&tags->lock);

        return tag;
}

/**
 * percpu_ida_alloc - allocate a tag
 * @pool: pool to allocate from
 * @state: task state for prepare_to_wait
 *
 * Returns a tag - an integer in the range [0..nr_tags) (passed to
 * tag_pool_init()), or otherwise -ENOSPC on allocation failure.
 *
 * Safe to be called from interrupt context (assuming it isn't passed
 * TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE, of course).
 *
 * @gfp indicates whether or not to wait until a free id is available (it's not
 * used for internal memory allocations); thus if passed __GFP_RECLAIM we may sleep
 * however long it takes until another thread frees an id (same semantics as a
 * mempool).
 *
 * Will not fail if passed TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE.
 */
int percpu_ida_alloc(struct percpu_ida *pool, int state)
{
        DEFINE_WAIT(wait);
        struct percpu_ida_cpu *tags;
        unsigned long flags;
        int tag;

        local_irq_save(flags);
        tags = this_cpu_ptr(pool->tag_cpu);

        /* Fastpath */
        tag = alloc_local_tag(tags);
        if (likely(tag >= 0)) {
                local_irq_restore(flags);
                return tag;
        }

        while (1) {
                spin_lock(&pool->lock);

                /*
                 * prepare_to_wait() must come before steal_tags(), in case
                 * percpu_ida_free() on another cpu flips a bit in
                 * cpus_have_tags
                 *
                 * global lock held and irqs disabled, don't need percpu lock
                 */
                if (state != TASK_RUNNING)
                        prepare_to_wait(&pool->wait, &wait, state);

                if (!tags->nr_free)
                        alloc_global_tags(pool, tags);
                if (!tags->nr_free)
                        steal_tags(pool, tags);

                if (tags->nr_free) {
                        tag = tags->freelist[--tags->nr_free];
                        if (tags->nr_free)
                                cpumask_set_cpu(smp_processor_id(),
                                                &pool->cpus_have_tags);
                }

                spin_unlock(&pool->lock);
                local_irq_restore(flags);

                if (tag >= 0 || state == TASK_RUNNING)
                        break;

                if (signal_pending_state(state, current)) {
                        tag = -ERESTARTSYS;
                        break;
                }

                schedule();

                local_irq_save(flags);
                tags = this_cpu_ptr(pool->tag_cpu);
        }
        if (state != TASK_RUNNING)
                finish_wait(&pool->wait, &wait);

        return tag;
}
EXPORT_SYMBOL_GPL(percpu_ida_alloc);

/**
 * percpu_ida_free - free a tag
 * @pool: pool @tag was allocated from
 * @tag: a tag previously allocated with percpu_ida_alloc()
 *
 * Safe to be called from interrupt context.
 */
void percpu_ida_free(struct percpu_ida *pool, unsigned tag)
{
        struct percpu_ida_cpu *tags;
        unsigned long flags;
        unsigned nr_free;

        BUG_ON(tag >= pool->nr_tags);

        local_irq_save(flags);
        tags = this_cpu_ptr(pool->tag_cpu);

        spin_lock(&tags->lock);
        tags->freelist[tags->nr_free++] = tag;

        nr_free = tags->nr_free;
        spin_unlock(&tags->lock);

        if (nr_free == 1) {
                cpumask_set_cpu(smp_processor_id(),
                                &pool->cpus_have_tags);
                wake_up(&pool->wait);
        }

        if (nr_free == pool->percpu_max_size) {
                spin_lock(&pool->lock);

                /*
                 * Global lock held and irqs disabled, don't need percpu
                 * lock
                 */
                if (tags->nr_free == pool->percpu_max_size) {
                        move_tags(pool->freelist, &pool->nr_free,
                                  tags->freelist, &tags->nr_free,
                                  pool->percpu_batch_size);

                        wake_up(&pool->wait);
                }
                spin_unlock(&pool->lock);
        }

        local_irq_restore(flags);
}
EXPORT_SYMBOL_GPL(percpu_ida_free);

/**
 * percpu_ida_destroy - release a tag pool's resources
 * @pool: pool to free
 *
 * Frees the resources allocated by percpu_ida_init().
 */
void percpu_ida_destroy(struct percpu_ida *pool)
{
        free_percpu(pool->tag_cpu);
        free_pages((unsigned long) pool->freelist,
                   get_order(pool->nr_tags * sizeof(unsigned)));
}
EXPORT_SYMBOL_GPL(percpu_ida_destroy);

/**
 * percpu_ida_init - initialize a percpu tag pool
 * @pool: pool to initialize
 * @nr_tags: number of tags that will be available for allocation
 *
 * Initializes @pool so that it can be used to allocate tags - integers in the
 * range [0, nr_tags). Typically, they'll be used by driver code to refer to a
 * preallocated array of tag structures.
 *
 * Allocation is percpu, but sharding is limited by nr_tags - for best
 * performance, the workload should not span more cpus than nr_tags / 128.
 */
int __percpu_ida_init(struct percpu_ida *pool, unsigned long nr_tags,
        unsigned long max_size, unsigned long batch_size)
{
        unsigned i, cpu, order;

        memset(pool, 0, sizeof(*pool));

        init_waitqueue_head(&pool->wait);
        spin_lock_init(&pool->lock);
        pool->nr_tags = nr_tags;
        pool->percpu_max_size = max_size;
        pool->percpu_batch_size = batch_size;

        /* Guard against overflow */
        if (nr_tags > (unsigned) INT_MAX + 1) {
                pr_err("percpu_ida_init(): nr_tags too large\n");
                return -EINVAL;
        }

        order = get_order(nr_tags * sizeof(unsigned));
        pool->freelist = (void *) __get_free_pages(GFP_KERNEL, order);
        if (!pool->freelist)
                return -ENOMEM;

        for (i = 0; i < nr_tags; i++)
                pool->freelist[i] = i;

        pool->nr_free = nr_tags;

        pool->tag_cpu = __alloc_percpu(sizeof(struct percpu_ida_cpu) +
                                       pool->percpu_max_size * sizeof(unsigned),
                                       sizeof(unsigned));
        if (!pool->tag_cpu)
                goto err;

        for_each_possible_cpu(cpu)
                spin_lock_init(&per_cpu_ptr(pool->tag_cpu, cpu)->lock);

        return 0;
err:
        percpu_ida_destroy(pool);
        return -ENOMEM;
}
EXPORT_SYMBOL_GPL(__percpu_ida_init);

/**
 * percpu_ida_for_each_free - iterate free ids of a pool
 * @pool: pool to iterate
 * @fn: interate callback function
 * @data: parameter for @fn
 *
 * Note, this doesn't guarantee to iterate all free ids restrictly. Some free
 * ids might be missed, some might be iterated duplicated, and some might
 * be iterated and not free soon.
 */
int percpu_ida_for_each_free(struct percpu_ida *pool, percpu_ida_cb fn,
        void *data)
{
        unsigned long flags;
        struct percpu_ida_cpu *remote;
        unsigned cpu, i, err = 0;

        local_irq_save(flags);
        for_each_possible_cpu(cpu) {
                remote = per_cpu_ptr(pool->tag_cpu, cpu);
                spin_lock(&remote->lock);
                for (i = 0; i < remote->nr_free; i++) {
                        err = fn(remote->freelist[i], data);
                        if (err)
                                break;
                }
                spin_unlock(&remote->lock);
                if (err)
                        goto out;
        }

        spin_lock(&pool->lock);
        for (i = 0; i < pool->nr_free; i++) {
                err = fn(pool->freelist[i], data);
                if (err)
                        break;
        }
        spin_unlock(&pool->lock);
out:
        local_irq_restore(flags);
        return err;
}
EXPORT_SYMBOL_GPL(percpu_ida_for_each_free);

/**
 * percpu_ida_free_tags - return free tags number of a specific cpu or global pool
 * @pool: pool related
 * @cpu: specific cpu or global pool if @cpu == nr_cpu_ids
 *
 * Note: this just returns a snapshot of free tags number.
 */
unsigned percpu_ida_free_tags(struct percpu_ida *pool, int cpu)
{
        struct percpu_ida_cpu *remote;
        if (cpu == nr_cpu_ids)
                return pool->nr_free;
        remote = per_cpu_ptr(pool->tag_cpu, cpu);
        return remote->nr_free;
}
EXPORT_SYMBOL_GPL(percpu_ida_free_tags);