/*
   * 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/hardirq.h>
  #include <linux/idr.h>
  #include <linux/init.h>
  #include <linux/kernel.h>
  #include <linux/percpu.h>
  #include <linux/sched.h>
  #include <linux/slab.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_WAIT 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
  ");
  		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);