async-thread.c

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/*
 * Copyright (C) 2007 Oracle.  All rights reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public
 * License v2 as published by the Free Software Foundation.
 *
 * 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.
 *
 * You should have received a copy of the GNU General Public
 * License along with this program; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 021110-1307, USA.
 */

#include <linux/kthread.h>
#include <linux/slab.h>
#include <linux/list.h>
#include <linux/spinlock.h>
#include <linux/freezer.h>
#include "async-thread.h"

#define WORK_QUEUED_BIT 0
#define WORK_DONE_BIT 1
#define WORK_ORDER_DONE_BIT 2
#define WORK_HIGH_PRIO_BIT 3

/*
 * container for the kthread task pointer and the list of pending work
 * One of these is allocated per thread.
 */
struct btrfs_worker_thread {
    /* pool we belong to */
    struct btrfs_workers *workers;

    /* list of struct btrfs_work that are waiting for service */
    struct list_head pending;
    struct list_head prio_pending;

    /* list of worker threads from struct btrfs_workers */
    struct list_head worker_list;

    /* kthread */
    struct task_struct *task;

    /* number of things on the pending list */
    atomic_t num_pending;

    /* reference counter for this struct */
    atomic_t refs;

    unsigned long sequence;

    /* protects the pending list. */
    spinlock_t lock;

    /* set to non-zero when this thread is already awake and kicking */
    int working;

    /* are we currently idle */
    int idle;
};

static int __btrfs_start_workers(struct btrfs_workers *workers);

/*
 * btrfs_start_workers uses kthread_run, which can block waiting for memory
 * for a very long time.  It will actually throttle on page writeback,
 * and so it may not make progress until after our btrfs worker threads
 * process all of the pending work structs in their queue
 *
 * This means we can't use btrfs_start_workers from inside a btrfs worker
 * thread that is used as part of cleaning dirty memory, which pretty much
 * involves all of the worker threads.
 *
 * Instead we have a helper queue who never has more than one thread
 * where we scheduler thread start operations.  This worker_start struct
 * is used to contain the work and hold a pointer to the queue that needs
 * another worker.
 */
struct worker_start {
    struct btrfs_work work;
    struct btrfs_workers *queue;
};

static void start_new_worker_func(struct btrfs_work *work)
{
    struct worker_start *start;
    start = container_of(work, struct worker_start, work);
    __btrfs_start_workers(start->queue);
    kfree(start);
}

/*
 * helper function to move a thread onto the idle list after it
 * has finished some requests.
 */
static void check_idle_worker(struct btrfs_worker_thread *worker)
{
    if (!worker->idle && atomic_read(&worker->num_pending) <
        worker->workers->idle_thresh / 2) {
        unsigned long flags;
        spin_lock_irqsave(&worker->workers->lock, flags);
        worker->idle = 1;

        /* the list may be empty if the worker is just starting */
        if (!list_empty(&worker->worker_list)) {
            list_move(&worker->worker_list,
                 &worker->workers->idle_list);
        }
        spin_unlock_irqrestore(&worker->workers->lock, flags);
    }
}

/*
 * helper function to move a thread off the idle list after new
 * pending work is added.
 */
static void check_busy_worker(struct btrfs_worker_thread *worker)
{
    if (worker->idle && atomic_read(&worker->num_pending) >=
        worker->workers->idle_thresh) {
        unsigned long flags;
        spin_lock_irqsave(&worker->workers->lock, flags);
        worker->idle = 0;

        if (!list_empty(&worker->worker_list)) {
            list_move_tail(&worker->worker_list,
                      &worker->workers->worker_list);
        }
        spin_unlock_irqrestore(&worker->workers->lock, flags);
    }
}

static void check_pending_worker_creates(struct btrfs_worker_thread *worker)
{
    struct btrfs_workers *workers = worker->workers;
    struct worker_start *start;
    unsigned long flags;

    rmb();
    if (!workers->atomic_start_pending)
        return;

    start = kzalloc(sizeof(*start), GFP_NOFS);
    if (!start)
        return;

    start->work.func = start_new_worker_func;
    start->queue = workers;

    spin_lock_irqsave(&workers->lock, flags);
    if (!workers->atomic_start_pending)
        goto out;

    workers->atomic_start_pending = 0;
    if (workers->num_workers + workers->num_workers_starting >=
        workers->max_workers)
        goto out;

    workers->num_workers_starting += 1;
    spin_unlock_irqrestore(&workers->lock, flags);
    btrfs_queue_worker(workers->atomic_worker_start, &start->work);
    return;

out:
    kfree(start);
    spin_unlock_irqrestore(&workers->lock, flags);
}

static noinline void run_ordered_completions(struct btrfs_workers *workers,
                        struct btrfs_work *work)
{
    if (!workers->ordered)
        return;

    set_bit(WORK_DONE_BIT, &work->flags);

    spin_lock(&workers->order_lock);

    while (1) {
        if (!list_empty(&workers->prio_order_list)) {
            work = list_entry(workers->prio_order_list.next,
                      struct btrfs_work, order_list);
        } else if (!list_empty(&workers->order_list)) {
            work = list_entry(workers->order_list.next,
                      struct btrfs_work, order_list);
        } else {
            break;
        }
        if (!test_bit(WORK_DONE_BIT, &work->flags))
            break;

        /* we are going to call the ordered done function, but
         * we leave the work item on the list as a barrier so
         * that later work items that are done don't have their
         * functions called before this one returns
         */
        if (test_and_set_bit(WORK_ORDER_DONE_BIT, &work->flags))
            break;

        spin_unlock(&workers->order_lock);

        work->ordered_func(work);

        /* now take the lock again and drop our item from the list */
        spin_lock(&workers->order_lock);
        list_del(&work->order_list);
        spin_unlock(&workers->order_lock);

        /*
         * we don't want to call the ordered free functions
         * with the lock held though
         */
        work->ordered_free(work);
        spin_lock(&workers->order_lock);
    }

    spin_unlock(&workers->order_lock);
}

static void put_worker(struct btrfs_worker_thread *worker)
{
    if (atomic_dec_and_test(&worker->refs))
        kfree(worker);
}

static int try_worker_shutdown(struct btrfs_worker_thread *worker)
{
    int freeit = 0;

    spin_lock_irq(&worker->lock);
    spin_lock(&worker->workers->lock);
    if (worker->workers->num_workers > 1 &&
        worker->idle &&
        !worker->working &&
        !list_empty(&worker->worker_list) &&
        list_empty(&worker->prio_pending) &&
        list_empty(&worker->pending) &&
        atomic_read(&worker->num_pending) == 0) {
        freeit = 1;
        list_del_init(&worker->worker_list);
        worker->workers->num_workers--;
    }
    spin_unlock(&worker->workers->lock);
    spin_unlock_irq(&worker->lock);

    if (freeit)
        put_worker(worker);
    return freeit;
}

static struct btrfs_work *get_next_work(struct btrfs_worker_thread *worker,
                    struct list_head *prio_head,
                    struct list_head *head)
{
    struct btrfs_work *work = NULL;
    struct list_head *cur = NULL;

    if(!list_empty(prio_head))
        cur = prio_head->next;

    smp_mb();
    if (!list_empty(&worker->prio_pending))
        goto refill;

    if (!list_empty(head))
        cur = head->next;

    if (cur)
        goto out;

refill:
    spin_lock_irq(&worker->lock);
    list_splice_tail_init(&worker->prio_pending, prio_head);
    list_splice_tail_init(&worker->pending, head);

    if (!list_empty(prio_head))
        cur = prio_head->next;
    else if (!list_empty(head))
        cur = head->next;
    spin_unlock_irq(&worker->lock);

    if (!cur)
        goto out_fail;

out:
    work = list_entry(cur, struct btrfs_work, list);

out_fail:
    return work;
}

/*
 * main loop for servicing work items
 */
static int worker_loop(void *arg)
{
    struct btrfs_worker_thread *worker = arg;
    struct list_head head;
    struct list_head prio_head;
    struct btrfs_work *work;

    INIT_LIST_HEAD(&head);
    INIT_LIST_HEAD(&prio_head);

    do {
again:
        while (1) {


            work = get_next_work(worker, &prio_head, &head);
            if (!work)
                break;

            list_del(&work->list);
            clear_bit(WORK_QUEUED_BIT, &work->flags);

            work->worker = worker;

            work->func(work);

            atomic_dec(&worker->num_pending);
            /*
             * unless this is an ordered work queue,
             * 'work' was probably freed by func above.
             */
            run_ordered_completions(worker->workers, work);

            check_pending_worker_creates(worker);
            cond_resched();
        }

        spin_lock_irq(&worker->lock);
        check_idle_worker(worker);

        if (freezing(current)) {
            worker->working = 0;
            spin_unlock_irq(&worker->lock);
            try_to_freeze();
        } else {
            spin_unlock_irq(&worker->lock);
            if (!kthread_should_stop()) {
                cpu_relax();
                /*
                 * we've dropped the lock, did someone else
                 * jump_in?
                 */
                smp_mb();
                if (!list_empty(&worker->pending) ||
                    !list_empty(&worker->prio_pending))
                    continue;

                /*
                 * this short schedule allows more work to
                 * come in without the queue functions
                 * needing to go through wake_up_process()
                 *
                 * worker->working is still 1, so nobody
                 * is going to try and wake us up
                 */
                schedule_timeout(1);
                smp_mb();
                if (!list_empty(&worker->pending) ||
                    !list_empty(&worker->prio_pending))
                    continue;

                if (kthread_should_stop())
                    break;

                /* still no more work?, sleep for real */
                spin_lock_irq(&worker->lock);
                set_current_state(TASK_INTERRUPTIBLE);
                if (!list_empty(&worker->pending) ||
                    !list_empty(&worker->prio_pending)) {
                    spin_unlock_irq(&worker->lock);
                    set_current_state(TASK_RUNNING);
                    goto again;
                }

                /*
                 * this makes sure we get a wakeup when someone
                 * adds something new to the queue
                 */
                worker->working = 0;
                spin_unlock_irq(&worker->lock);

                if (!kthread_should_stop()) {
                    schedule_timeout(HZ * 120);
                    if (!worker->working &&
                        try_worker_shutdown(worker)) {
                        return 0;
                    }
                }
            }
            __set_current_state(TASK_RUNNING);
        }
    } while (!kthread_should_stop());
    return 0;
}

/*
 * this will wait for all the worker threads to shutdown
 */
void btrfs_stop_workers(struct btrfs_workers *workers)
{
    struct list_head *cur;
    struct btrfs_worker_thread *worker;
    int can_stop;

    spin_lock_irq(&workers->lock);
    list_splice_init(&workers->idle_list, &workers->worker_list);
    while (!list_empty(&workers->worker_list)) {
        cur = workers->worker_list.next;
        worker = list_entry(cur, struct btrfs_worker_thread,
                    worker_list);

        atomic_inc(&worker->refs);
        workers->num_workers -= 1;
        if (!list_empty(&worker->worker_list)) {
            list_del_init(&worker->worker_list);
            put_worker(worker);
            can_stop = 1;
        } else
            can_stop = 0;
        spin_unlock_irq(&workers->lock);
        if (can_stop)
            kthread_stop(worker->task);
        spin_lock_irq(&workers->lock);
        put_worker(worker);
    }
    spin_unlock_irq(&workers->lock);
}

/*
 * simple init on struct btrfs_workers
 */
void btrfs_init_workers(struct btrfs_workers *workers, char *name, int max,
            struct btrfs_workers *async_helper)
{
    workers->num_workers = 0;
    workers->num_workers_starting = 0;
    INIT_LIST_HEAD(&workers->worker_list);
    INIT_LIST_HEAD(&workers->idle_list);
    INIT_LIST_HEAD(&workers->order_list);
    INIT_LIST_HEAD(&workers->prio_order_list);
    spin_lock_init(&workers->lock);
    spin_lock_init(&workers->order_lock);
    workers->max_workers = max;
    workers->idle_thresh = 32;
    workers->name = name;
    workers->ordered = 0;
    workers->atomic_start_pending = 0;
    workers->atomic_worker_start = async_helper;
}

/*
 * starts new worker threads.  This does not enforce the max worker
 * count in case you need to temporarily go past it.
 */
static int __btrfs_start_workers(struct btrfs_workers *workers)
{
    struct btrfs_worker_thread *worker;
    int ret = 0;

    worker = kzalloc(sizeof(*worker), GFP_NOFS);
    if (!worker) {
        ret = -ENOMEM;
        goto fail;
    }

    INIT_LIST_HEAD(&worker->pending);
    INIT_LIST_HEAD(&worker->prio_pending);
    INIT_LIST_HEAD(&worker->worker_list);
    spin_lock_init(&worker->lock);

    atomic_set(&worker->num_pending, 0);
    atomic_set(&worker->refs, 1);
    worker->workers = workers;
    worker->task = kthread_run(worker_loop, worker,
                   "btrfs-%s-%d", workers->name,
                   workers->num_workers + 1);
    if (IS_ERR(worker->task)) {
        ret = PTR_ERR(worker->task);
        kfree(worker);
        goto fail;
    }
    spin_lock_irq(&workers->lock);
    list_add_tail(&worker->worker_list, &workers->idle_list);
    worker->idle = 1;
    workers->num_workers++;
    workers->num_workers_starting--;
    WARN_ON(workers->num_workers_starting < 0);
    spin_unlock_irq(&workers->lock);

    return 0;
fail:
    spin_lock_irq(&workers->lock);
    workers->num_workers_starting--;
    spin_unlock_irq(&workers->lock);
    return ret;
}

int btrfs_start_workers(struct btrfs_workers *workers)
{
    spin_lock_irq(&workers->lock);
    workers->num_workers_starting++;
    spin_unlock_irq(&workers->lock);
    return __btrfs_start_workers(workers);
}

/*
 * run through the list and find a worker thread that doesn't have a lot
 * to do right now.  This can return null if we aren't yet at the thread
 * count limit and all of the threads are busy.
 */
static struct btrfs_worker_thread *next_worker(struct btrfs_workers *workers)
{
    struct btrfs_worker_thread *worker;
    struct list_head *next;
    int enforce_min;

    enforce_min = (workers->num_workers + workers->num_workers_starting) <
        workers->max_workers;

    /*
     * if we find an idle thread, don't move it to the end of the
     * idle list.  This improves the chance that the next submission
     * will reuse the same thread, and maybe catch it while it is still
     * working
     */
    if (!list_empty(&workers->idle_list)) {
        next = workers->idle_list.next;
        worker = list_entry(next, struct btrfs_worker_thread,
                    worker_list);
        return worker;
    }
    if (enforce_min || list_empty(&workers->worker_list))
        return NULL;

    /*
     * if we pick a busy task, move the task to the end of the list.
     * hopefully this will keep things somewhat evenly balanced.
     * Do the move in batches based on the sequence number.  This groups
     * requests submitted at roughly the same time onto the same worker.
     */
    next = workers->worker_list.next;
    worker = list_entry(next, struct btrfs_worker_thread, worker_list);
    worker->sequence++;

    if (worker->sequence % workers->idle_thresh == 0)
        list_move_tail(next, &workers->worker_list);
    return worker;
}

/*
 * selects a worker thread to take the next job.  This will either find
 * an idle worker, start a new worker up to the max count, or just return
 * one of the existing busy workers.
 */
static struct btrfs_worker_thread *find_worker(struct btrfs_workers *workers)
{
    struct btrfs_worker_thread *worker;
    unsigned long flags;
    struct list_head *fallback;
    int ret;

    spin_lock_irqsave(&workers->lock, flags);
again:
    worker = next_worker(workers);

    if (!worker) {
        if (workers->num_workers + workers->num_workers_starting >=
            workers->max_workers) {
            goto fallback;
        } else if (workers->atomic_worker_start) {
            workers->atomic_start_pending = 1;
            goto fallback;
        } else {
            workers->num_workers_starting++;
            spin_unlock_irqrestore(&workers->lock, flags);
            /* we're below the limit, start another worker */
            ret = __btrfs_start_workers(workers);
            spin_lock_irqsave(&workers->lock, flags);
            if (ret)
                goto fallback;
            goto again;
        }
    }
    goto found;

fallback:
    fallback = NULL;
    /*
     * we have failed to find any workers, just
     * return the first one we can find.
     */
    if (!list_empty(&workers->worker_list))
        fallback = workers->worker_list.next;
    if (!list_empty(&workers->idle_list))
        fallback = workers->idle_list.next;
    BUG_ON(!fallback);
    worker = list_entry(fallback,
          struct btrfs_worker_thread, worker_list);
found:
    /*
     * this makes sure the worker doesn't exit before it is placed
     * onto a busy/idle list
     */
    atomic_inc(&worker->num_pending);
    spin_unlock_irqrestore(&workers->lock, flags);
    return worker;
}

/*
 * btrfs_requeue_work just puts the work item back on the tail of the list
 * it was taken from.  It is intended for use with long running work functions
 * that make some progress and want to give the cpu up for others.
 */
void btrfs_requeue_work(struct btrfs_work *work)
{
    struct btrfs_worker_thread *worker = work->worker;
    unsigned long flags;
    int wake = 0;

    if (test_and_set_bit(WORK_QUEUED_BIT, &work->flags))
        return;

    spin_lock_irqsave(&worker->lock, flags);
    if (test_bit(WORK_HIGH_PRIO_BIT, &work->flags))
        list_add_tail(&work->list, &worker->prio_pending);
    else
        list_add_tail(&work->list, &worker->pending);
    atomic_inc(&worker->num_pending);

    /* by definition we're busy, take ourselves off the idle
     * list
     */
    if (worker->idle) {
        spin_lock(&worker->workers->lock);
        worker->idle = 0;
        list_move_tail(&worker->worker_list,
                  &worker->workers->worker_list);
        spin_unlock(&worker->workers->lock);
    }
    if (!worker->working) {
        wake = 1;
        worker->working = 1;
    }

    if (wake)
        wake_up_process(worker->task);
    spin_unlock_irqrestore(&worker->lock, flags);
}

void btrfs_set_work_high_prio(struct btrfs_work *work)
{
    set_bit(WORK_HIGH_PRIO_BIT, &work->flags);
}

/*
 * places a struct btrfs_work into the pending queue of one of the kthreads
 */
void btrfs_queue_worker(struct btrfs_workers *workers, struct btrfs_work *work)
{
    struct btrfs_worker_thread *worker;
    unsigned long flags;
    int wake = 0;

    /* don't requeue something already on a list */
    if (test_and_set_bit(WORK_QUEUED_BIT, &work->flags))
        return;

    worker = find_worker(workers);
    if (workers->ordered) {
        /*
         * you're not allowed to do ordered queues from an
         * interrupt handler
         */
        spin_lock(&workers->order_lock);
        if (test_bit(WORK_HIGH_PRIO_BIT, &work->flags)) {
            list_add_tail(&work->order_list,
                      &workers->prio_order_list);
        } else {
            list_add_tail(&work->order_list, &workers->order_list);
        }
        spin_unlock(&workers->order_lock);
    } else {
        INIT_LIST_HEAD(&work->order_list);
    }

    spin_lock_irqsave(&worker->lock, flags);

    if (test_bit(WORK_HIGH_PRIO_BIT, &work->flags))
        list_add_tail(&work->list, &worker->prio_pending);
    else
        list_add_tail(&work->list, &worker->pending);
    check_busy_worker(worker);

    /*
     * avoid calling into wake_up_process if this thread has already
     * been kicked
     */
    if (!worker->working)
        wake = 1;
    worker->working = 1;

    if (wake)
        wake_up_process(worker->task);
    spin_unlock_irqrestore(&worker->lock, flags);
}