/*
   * mm/pdflush.c - worker threads for writing back filesystem data
   *
   * Copyright (C) 2002, Linus Torvalds.
   *
   * 09Apr2002	akpm@zip.com.au
   *		Initial version
   * 29Feb2004	kaos@sgi.com
   *		Move worker thread creation to kthread to avoid chewing
   *		up stack space with nested calls to kernel_thread.
   */
  
  #include <linux/sched.h>
  #include <linux/list.h>
  #include <linux/signal.h>
  #include <linux/spinlock.h>
  #include <linux/gfp.h>
  #include <linux/init.h>
  #include <linux/module.h>

  #include <linux/fs.h>		/* Needed by writeback.h	  */
  #include <linux/writeback.h>	/* Prototypes pdflush_operation() */

  #include <linux/kthread.h>

  #include <linux/cpuset.h>

  #include <linux/freezer.h>

  
  
  /*
   * Minimum and maximum number of pdflush instances
   */
  #define MIN_PDFLUSH_THREADS	2
  #define MAX_PDFLUSH_THREADS	8
  
  static void start_one_pdflush_thread(void);
  
  
  /*
   * The pdflush threads are worker threads for writing back dirty data.
   * Ideally, we'd like one thread per active disk spindle.  But the disk
   * topology is very hard to divine at this level.   Instead, we take
   * care in various places to prevent more than one pdflush thread from
   * performing writeback against a single filesystem.  pdflush threads
   * have the PF_FLUSHER flag set in current->flags to aid in this.
   */
  
  /*
   * All the pdflush threads.  Protected by pdflush_lock
   */
  static LIST_HEAD(pdflush_list);
  static DEFINE_SPINLOCK(pdflush_lock);
  
  /*
   * The count of currently-running pdflush threads.  Protected
   * by pdflush_lock.
   *
   * Readable by sysctl, but not writable.  Published to userspace at
   * /proc/sys/vm/nr_pdflush_threads.
   */
  int nr_pdflush_threads = 0;
  
  /*
   * The time at which the pdflush thread pool last went empty
   */
  static unsigned long last_empty_jifs;
  
  /*
   * The pdflush thread.
   *
   * Thread pool management algorithm:
   * 
   * - The minimum and maximum number of pdflush instances are bound
   *   by MIN_PDFLUSH_THREADS and MAX_PDFLUSH_THREADS.
   * 
   * - If there have been no idle pdflush instances for 1 second, create
   *   a new one.
   * 
   * - If the least-recently-went-to-sleep pdflush thread has been asleep
   *   for more than one second, terminate a thread.
   */
  
  /*
   * A structure for passing work to a pdflush thread.  Also for passing
   * state information between pdflush threads.  Protected by pdflush_lock.
   */
  struct pdflush_work {
  	struct task_struct *who;	/* The thread */
  	void (*fn)(unsigned long);	/* A callback function */
  	unsigned long arg0;		/* An argument to the callback */
  	struct list_head list;		/* On pdflush_list, when idle */
  	unsigned long when_i_went_to_sleep;
  };
  
  static int __pdflush(struct pdflush_work *my_work)
  {

  	current->flags |= PF_FLUSHER | PF_SWAPWRITE;

  	set_freezable();

  	my_work->fn = NULL;
  	my_work->who = current;
  	INIT_LIST_HEAD(&my_work->list);
  
  	spin_lock_irq(&pdflush_lock);
  	nr_pdflush_threads++;
  	for ( ; ; ) {
  		struct pdflush_work *pdf;
  
  		set_current_state(TASK_INTERRUPTIBLE);
  		list_move(&my_work->list, &pdflush_list);
  		my_work->when_i_went_to_sleep = jiffies;
  		spin_unlock_irq(&pdflush_lock);

  		schedule();

  		try_to_freeze();

  		spin_lock_irq(&pdflush_lock);
  		if (!list_empty(&my_work->list)) {

  			/*
  			 * Someone woke us up, but without removing our control
  			 * structure from the global list.  swsusp will do this
  			 * in try_to_freeze()->refrigerator().  Handle it.
  			 */

  			my_work->fn = NULL;
  			continue;
  		}
  		if (my_work->fn == NULL) {

  			printk("pdflush: bogus wakeup
  ");

  			continue;
  		}
  		spin_unlock_irq(&pdflush_lock);
  
  		(*my_work->fn)(my_work->arg0);
  
  		/*
  		 * Thread creation: For how long have there been zero
  		 * available threads?
  		 */
  		if (jiffies - last_empty_jifs > 1 * HZ) {
  			/* unlocked list_empty() test is OK here */
  			if (list_empty(&pdflush_list)) {
  				/* unlocked test is OK here */
  				if (nr_pdflush_threads < MAX_PDFLUSH_THREADS)
  					start_one_pdflush_thread();
  			}
  		}
  
  		spin_lock_irq(&pdflush_lock);
  		my_work->fn = NULL;
  
  		/*
  		 * Thread destruction: For how long has the sleepiest
  		 * thread slept?
  		 */
  		if (list_empty(&pdflush_list))
  			continue;
  		if (nr_pdflush_threads <= MIN_PDFLUSH_THREADS)
  			continue;
  		pdf = list_entry(pdflush_list.prev, struct pdflush_work, list);
  		if (jiffies - pdf->when_i_went_to_sleep > 1 * HZ) {
  			/* Limit exit rate */
  			pdf->when_i_went_to_sleep = jiffies;
  			break;					/* exeunt */
  		}
  	}
  	nr_pdflush_threads--;
  	spin_unlock_irq(&pdflush_lock);
  	return 0;
  }
  
  /*
   * Of course, my_work wants to be just a local in __pdflush().  It is
   * separated out in this manner to hopefully prevent the compiler from
   * performing unfortunate optimisations against the auto variables.  Because
   * these are visible to other tasks and CPUs.  (No problem has actually
   * been observed.  This is just paranoia).
   */
  static int pdflush(void *dummy)
  {
  	struct pdflush_work my_work;

  	cpumask_t cpus_allowed;

  
  	/*
  	 * pdflush can spend a lot of time doing encryption via dm-crypt.  We
  	 * don't want to do that at keventd's priority.
  	 */
  	set_user_nice(current, 0);

  
  	/*
  	 * Some configs put our parent kthread in a limited cpuset,
  	 * which kthread() overrides, forcing cpus_allowed == CPU_MASK_ALL.
  	 * Our needs are more modest - cut back to our cpusets cpus_allowed.
  	 * This is needed as pdflush's are dynamically created and destroyed.
  	 * The boottime pdflush's are easily placed w/o these 2 lines.
  	 */

  	cpuset_cpus_allowed(current, &cpus_allowed);
  	set_cpus_allowed_ptr(current, &cpus_allowed);

  	return __pdflush(&my_work);
  }
  
  /*
   * Attempt to wake up a pdflush thread, and get it to do some work for you.
   * Returns zero if it indeed managed to find a worker thread, and passed your
   * payload to it.
   */
  int pdflush_operation(void (*fn)(unsigned long), unsigned long arg0)
  {
  	unsigned long flags;
  	int ret = 0;

  	BUG_ON(fn == NULL);	/* Hard to diagnose if it's deferred */

  
  	spin_lock_irqsave(&pdflush_lock, flags);
  	if (list_empty(&pdflush_list)) {

  		ret = -1;
  	} else {
  		struct pdflush_work *pdf;
  
  		pdf = list_entry(pdflush_list.next, struct pdflush_work, list);
  		list_del_init(&pdf->list);
  		if (list_empty(&pdflush_list))
  			last_empty_jifs = jiffies;
  		pdf->fn = fn;
  		pdf->arg0 = arg0;
  		wake_up_process(pdf->who);

  	}

  	spin_unlock_irqrestore(&pdflush_lock, flags);

  	return ret;
  }
  
  static void start_one_pdflush_thread(void)
  {
  	kthread_run(pdflush, NULL, "pdflush");
  }
  
  static int __init pdflush_init(void)
  {
  	int i;
  
  	for (i = 0; i < MIN_PDFLUSH_THREADS; i++)
  		start_one_pdflush_thread();
  	return 0;
  }
  
  module_init(pdflush_init);