/*
   * Copyright (C) 2004-2005 IBM Corp.  All Rights Reserved.
   * Copyright (C) 2006-2009 NEC Corporation.
   *
   * dm-queue-length.c
   *
   * Module Author: Stefan Bader, IBM
   * Modified by: Kiyoshi Ueda, NEC
   *
   * This file is released under the GPL.
   *
   * queue-length path selector - choose a path with the least number of
   * in-flight I/Os.
   */
  
  #include "dm.h"
  #include "dm-path-selector.h"
  
  #include <linux/slab.h>
  #include <linux/ctype.h>
  #include <linux/errno.h>
  #include <linux/module.h>
  #include <linux/atomic.h>
  
  #define DM_MSG_PREFIX	"multipath queue-length"
  #define QL_MIN_IO	128
  #define QL_VERSION	"0.1.0"
  
  struct selector {
  	struct list_head	valid_paths;
  	struct list_head	failed_paths;
  };
  
  struct path_info {
  	struct list_head	list;
  	struct dm_path		*path;
  	unsigned		repeat_count;
  	atomic_t		qlen;	/* the number of in-flight I/Os */
  };
  
  static struct selector *alloc_selector(void)
  {
  	struct selector *s = kmalloc(sizeof(*s), GFP_KERNEL);
  
  	if (s) {
  		INIT_LIST_HEAD(&s->valid_paths);
  		INIT_LIST_HEAD(&s->failed_paths);
  	}
  
  	return s;
  }
  
  static int ql_create(struct path_selector *ps, unsigned argc, char **argv)
  {
  	struct selector *s = alloc_selector();
  
  	if (!s)
  		return -ENOMEM;
  
  	ps->context = s;
  	return 0;
  }
  
  static void ql_free_paths(struct list_head *paths)
  {
  	struct path_info *pi, *next;
  
  	list_for_each_entry_safe(pi, next, paths, list) {
  		list_del(&pi->list);
  		kfree(pi);
  	}
  }
  
  static void ql_destroy(struct path_selector *ps)
  {
  	struct selector *s = ps->context;
  
  	ql_free_paths(&s->valid_paths);
  	ql_free_paths(&s->failed_paths);
  	kfree(s);
  	ps->context = NULL;
  }
  
  static int ql_status(struct path_selector *ps, struct dm_path *path,
  		     status_type_t type, char *result, unsigned maxlen)
  {
  	unsigned sz = 0;
  	struct path_info *pi;
  
  	/* When called with NULL path, return selector status/args. */
  	if (!path)
  		DMEMIT("0 ");
  	else {
  		pi = path->pscontext;
  
  		switch (type) {
  		case STATUSTYPE_INFO:
  			DMEMIT("%d ", atomic_read(&pi->qlen));
  			break;
  		case STATUSTYPE_TABLE:
  			DMEMIT("%u ", pi->repeat_count);
  			break;
  		}
  	}
  
  	return sz;
  }
  
  static int ql_add_path(struct path_selector *ps, struct dm_path *path,
  		       int argc, char **argv, char **error)
  {
  	struct selector *s = ps->context;
  	struct path_info *pi;
  	unsigned repeat_count = QL_MIN_IO;
  	char dummy;
  
  	/*
  	 * Arguments: [<repeat_count>]
  	 * 	<repeat_count>: The number of I/Os before switching path.
  	 * 			If not given, default (QL_MIN_IO) is used.
  	 */
  	if (argc > 1) {
  		*error = "queue-length ps: incorrect number of arguments";
  		return -EINVAL;
  	}
  
  	if ((argc == 1) && (sscanf(argv[0], "%u%c", &repeat_count, &dummy) != 1)) {
  		*error = "queue-length ps: invalid repeat count";
  		return -EINVAL;
  	}
  
  	/* Allocate the path information structure */
  	pi = kmalloc(sizeof(*pi), GFP_KERNEL);
  	if (!pi) {
  		*error = "queue-length ps: Error allocating path information";
  		return -ENOMEM;
  	}
  
  	pi->path = path;
  	pi->repeat_count = repeat_count;
  	atomic_set(&pi->qlen, 0);
  
  	path->pscontext = pi;
  
  	list_add_tail(&pi->list, &s->valid_paths);
  
  	return 0;
  }
  
  static void ql_fail_path(struct path_selector *ps, struct dm_path *path)
  {
  	struct selector *s = ps->context;
  	struct path_info *pi = path->pscontext;
  
  	list_move(&pi->list, &s->failed_paths);
  }
  
  static int ql_reinstate_path(struct path_selector *ps, struct dm_path *path)
  {
  	struct selector *s = ps->context;
  	struct path_info *pi = path->pscontext;
  
  	list_move_tail(&pi->list, &s->valid_paths);
  
  	return 0;
  }
  
  /*
   * Select a path having the minimum number of in-flight I/Os
   */
  static struct dm_path *ql_select_path(struct path_selector *ps,
  				      unsigned *repeat_count, size_t nr_bytes)
  {
  	struct selector *s = ps->context;
  	struct path_info *pi = NULL, *best = NULL;
  
  	if (list_empty(&s->valid_paths))
  		return NULL;
  
  	/* Change preferred (first in list) path to evenly balance. */
  	list_move_tail(s->valid_paths.next, &s->valid_paths);
  
  	list_for_each_entry(pi, &s->valid_paths, list) {
  		if (!best ||
  		    (atomic_read(&pi->qlen) < atomic_read(&best->qlen)))
  			best = pi;
  
  		if (!atomic_read(&best->qlen))
  			break;
  	}
  
  	if (!best)
  		return NULL;
  
  	*repeat_count = best->repeat_count;
  
  	return best->path;
  }
  
  static int ql_start_io(struct path_selector *ps, struct dm_path *path,
  		       size_t nr_bytes)
  {
  	struct path_info *pi = path->pscontext;
  
  	atomic_inc(&pi->qlen);
  
  	return 0;
  }
  
  static int ql_end_io(struct path_selector *ps, struct dm_path *path,
  		     size_t nr_bytes)
  {
  	struct path_info *pi = path->pscontext;
  
  	atomic_dec(&pi->qlen);
  
  	return 0;
  }
  
  static struct path_selector_type ql_ps = {
  	.name		= "queue-length",
  	.module		= THIS_MODULE,
  	.table_args	= 1,
  	.info_args	= 1,
  	.create		= ql_create,
  	.destroy	= ql_destroy,
  	.status		= ql_status,
  	.add_path	= ql_add_path,
  	.fail_path	= ql_fail_path,
  	.reinstate_path	= ql_reinstate_path,
  	.select_path	= ql_select_path,
  	.start_io	= ql_start_io,
  	.end_io		= ql_end_io,
  };
  
  static int __init dm_ql_init(void)
  {
  	int r = dm_register_path_selector(&ql_ps);
  
  	if (r < 0)
  		DMERR("register failed %d", r);
  
  	DMINFO("version " QL_VERSION " loaded");
  
  	return r;
  }
  
  static void __exit dm_ql_exit(void)
  {
  	int r = dm_unregister_path_selector(&ql_ps);
  
  	if (r < 0)
  		DMERR("unregister failed %d", r);
  }
  
  module_init(dm_ql_init);
  module_exit(dm_ql_exit);
  
  MODULE_AUTHOR("Stefan Bader <Stefan.Bader at de.ibm.com>");
  MODULE_DESCRIPTION(
  	"(C) Copyright IBM Corp. 2004,2005   All Rights Reserved.
  "
  	DM_NAME " path selector to balance the number of in-flight I/Os"
  );
  MODULE_LICENSE("GPL");