/*
 * 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>


/*
 * 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;
        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();
                if (try_to_freeze(PF_FREEZE)) {
                        spin_lock_irq(&pdflush_lock);
                        continue;
                }

                spin_lock_irq(&pdflush_lock);
                if (!list_empty(&my_work->list)) {
                        printk("pdflush: bogus wakeup!\n");
                        my_work->fn = NULL;
                        continue;
                }
                if (my_work->fn == NULL) {
                        printk("pdflush: NULL work function\n");
                        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;

        /*
         * 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);
        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;

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

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