[PATCH 35/35] Async thread routines

[Chris Mason <chris.mason@oracle.com>]

Btrfs users a number of helper threads to do cpu intensive tasks.  This
includes checksumming, compression and metadata updates that happen
after IO is complete.

This implementation uses kthreads and is intended to resize the thread
pool as it goes along.  Right now the pool grows but does not shrink.

A given thread pool can be flagged as ordered, which breaks the work
up into two phases.  The first phase is unordered work that can happen
in parallel (like compression) and the second phase is done in the
same order entries were put on the queue.

The ordered queues are used for submitting IO.  They allow asynchronous
work to be done while also preserving IO ordering and avoiding extra
seeks from parallel bio submission.

To limit context switches, each kthread tries to maintain a work list
of a given size (default is 64, some pools use other values).  The goal
is to keep a given thread busy without ping-ponging a small amount of work
between a large number of threads.

Btrfs uses a number of thread pools, mostly to prevent deadlocking between
them.  For example, the write IO completion functions may need to read
a metadata block, and so they must be done by separate thread pools.

Except for the word btrfs, these are fairly generic, and could be moved
outside of the btrfs directory.  The locking in them if fairly simple
and has room for optimization.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

diff --git a/fs/btrfs/async-thread.c b/fs/btrfs/async-thread.c
new file mode 100644
index 0000000..8e2fec0
--- /dev/null
+++ b/fs/btrfs/async-thread.c
@@ -0,0 +1,419 @@
+/*
+ * 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/version.h>
+#include <linux/kthread.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
+
+/*
+ * 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;
+
+	/* 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;
+
+	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;
+};
+
+/*
+ * 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;
+		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;
+		list_move_tail(&worker->worker_list,
+			       &worker->workers->worker_list);
+		spin_unlock_irqrestore(&worker->workers->lock, flags);
+	}
+}
+
+static noinline int run_ordered_completions(struct btrfs_workers *workers,
+					    struct btrfs_work *work)
+{
+	unsigned long flags;
+
+	if (!workers->ordered)
+		return 0;
+
+	set_bit(WORK_DONE_BIT, &work->flags);
+
+	spin_lock_irqsave(&workers->lock, flags);
+
+	while (!list_empty(&workers->order_list)) {
+		work = list_entry(workers->order_list.next,
+				  struct btrfs_work, order_list);
+
+		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_irqrestore(&workers->lock, flags);
+
+		work->ordered_func(work);
+
+		/* now take the lock again and call the freeing code */
+		spin_lock_irqsave(&workers->lock, flags);
+		list_del(&work->order_list);
+		work->ordered_free(work);
+	}
+
+	spin_unlock_irqrestore(&workers->lock, flags);
+	return 0;
+}
+
+/*
+ * main loop for servicing work items
+ */
+static int worker_loop(void *arg)
+{
+	struct btrfs_worker_thread *worker = arg;
+	struct list_head *cur;
+	struct btrfs_work *work;
+	do {
+		spin_lock_irq(&worker->lock);
+		while (!list_empty(&worker->pending)) {
+			cur = worker->pending.next;
+			work = list_entry(cur, struct btrfs_work, list);
+			list_del(&work->list);
+			clear_bit(WORK_QUEUED_BIT, &work->flags);
+
+			work->worker = worker;
+			spin_unlock_irq(&worker->lock);
+
+			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);
+
+			spin_lock_irq(&worker->lock);
+			check_idle_worker(worker);
+
+		}
+		worker->working = 0;
+		if (freezing(current)) {
+			refrigerator();
+		} else {
+			set_current_state(TASK_INTERRUPTIBLE);
+			spin_unlock_irq(&worker->lock);
+			if (!kthread_should_stop())
+				schedule();
+			__set_current_state(TASK_RUNNING);
+		}
+	} while (!kthread_should_stop());
+	return 0;
+}
+
+/*
+ * this will wait for all the worker threads to shutdown
+ */
+int btrfs_stop_workers(struct btrfs_workers *workers)
+{
+	struct list_head *cur;
+	struct btrfs_worker_thread *worker;
+
+	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);
+		kthread_stop(worker->task);
+		list_del(&worker->worker_list);
+		kfree(worker);
+	}
+	return 0;
+}
+
+/*
+ * simple init on struct btrfs_workers
+ */
+void btrfs_init_workers(struct btrfs_workers *workers, char *name, int max)
+{
+	workers->num_workers = 0;
+	INIT_LIST_HEAD(&workers->worker_list);
+	INIT_LIST_HEAD(&workers->idle_list);
+	INIT_LIST_HEAD(&workers->order_list);
+	spin_lock_init(&workers->lock);
+	workers->max_workers = max;
+	workers->idle_thresh = 32;
+	workers->name = name;
+	workers->ordered = 0;
+}
+
+/*
+ * starts new worker threads.  This does not enforce the max worker
+ * count in case you need to temporarily go past it.
+ */
+int btrfs_start_workers(struct btrfs_workers *workers, int num_workers)
+{
+	struct btrfs_worker_thread *worker;
+	int ret = 0;
+	int i;
+
+	for (i = 0; i < num_workers; i++) {
+		worker = kzalloc(sizeof(*worker), GFP_NOFS);
+		if (!worker) {
+			ret = -ENOMEM;
+			goto fail;
+		}
+
+		INIT_LIST_HEAD(&worker->pending);
+		INIT_LIST_HEAD(&worker->worker_list);
+		spin_lock_init(&worker->lock);
+		atomic_set(&worker->num_pending, 0);
+		worker->task = kthread_run(worker_loop, worker,
+					   "btrfs-%s-%d", workers->name,
+					   workers->num_workers + i);
+		worker->workers = workers;
+		if (IS_ERR(worker->task)) {
+			kfree(worker);
+			ret = PTR_ERR(worker->task);
+			goto fail;
+		}
+
+		spin_lock_irq(&workers->lock);
+		list_add_tail(&worker->worker_list, &workers->idle_list);
+		worker->idle = 1;
+		workers->num_workers++;
+		spin_unlock_irq(&workers->lock);
+	}
+	return 0;
+fail:
+	btrfs_stop_workers(workers);
+	return ret;
+}
+
+/*
+ * 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 = workers->num_workers < 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);
+	atomic_inc(&worker->num_pending);
+	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;
+
+again:
+	spin_lock_irqsave(&workers->lock, flags);
+	worker = next_worker(workers);
+	spin_unlock_irqrestore(&workers->lock, flags);
+
+	if (!worker) {
+		spin_lock_irqsave(&workers->lock, flags);
+		if (workers->num_workers >= workers->max_workers) {
+			struct list_head *fallback = NULL;
+			/*
+			 * we have failed to find any workers, just
+			 * return the force one
+			 */
+			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);
+			spin_unlock_irqrestore(&workers->lock, flags);
+		} else {
+			spin_unlock_irqrestore(&workers->lock, flags);
+			/* we're below the limit, start another worker */
+			btrfs_start_workers(workers, 1);
+			goto again;
+		}
+	}
+	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.
+ */
+int btrfs_requeue_work(struct btrfs_work *work)
+{
+	struct btrfs_worker_thread *worker = work->worker;
+	unsigned long flags;
+
+	if (test_and_set_bit(WORK_QUEUED_BIT, &work->flags))
+		goto out;
+
+	spin_lock_irqsave(&worker->lock, flags);
+	atomic_inc(&worker->num_pending);
+	list_add_tail(&work->list, &worker->pending);
+
+	/* by definition we're busy, take ourselves off the idle
+	 * list
+	 */
+	if (worker->idle) {
+		spin_lock_irqsave(&worker->workers->lock, flags);
+		worker->idle = 0;
+		list_move_tail(&worker->worker_list,
+			       &worker->workers->worker_list);
+		spin_unlock_irqrestore(&worker->workers->lock, flags);
+	}
+
+	spin_unlock_irqrestore(&worker->lock, flags);
+
+out:
+	return 0;
+}
+
+/*
+ * places a struct btrfs_work into the pending queue of one of the kthreads
+ */
+int 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))
+		goto out;
+
+	worker = find_worker(workers);
+	if (workers->ordered) {
+		spin_lock_irqsave(&workers->lock, flags);
+		list_add_tail(&work->order_list, &workers->order_list);
+		spin_unlock_irqrestore(&workers->lock, flags);
+	} else {
+		INIT_LIST_HEAD(&work->order_list);
+	}
+
+	spin_lock_irqsave(&worker->lock, flags);
+	atomic_inc(&worker->num_pending);
+	check_busy_worker(worker);
+	list_add_tail(&work->list, &worker->pending);
+
+	/*
+	 * avoid calling into wake_up_process if this thread has already
+	 * been kicked
+	 */
+	if (!worker->working)
+		wake = 1;
+	worker->working = 1;
+
+	spin_unlock_irqrestore(&worker->lock, flags);
+
+	if (wake)
+		wake_up_process(worker->task);
+out:
+	return 0;
+}
diff --git a/fs/btrfs/async-thread.h b/fs/btrfs/async-thread.h
new file mode 100644
index 0000000..31be4ed
--- /dev/null
+++ b/fs/btrfs/async-thread.h
@@ -0,0 +1,101 @@
+/*
+ * 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.
+ */
+
+#ifndef __BTRFS_ASYNC_THREAD_
+#define __BTRFS_ASYNC_THREAD_
+
+struct btrfs_worker_thread;
+
+/*
+ * This is similar to a workqueue, but it is meant to spread the operations
+ * across all available cpus instead of just the CPU that was used to
+ * queue the work.  There is also some batching introduced to try and
+ * cut down on context switches.
+ *
+ * By default threads are added on demand up to 2 * the number of cpus.
+ * Changing struct btrfs_workers->max_workers is one way to prevent
+ * demand creation of kthreads.
+ *
+ * the basic model of these worker threads is to embed a btrfs_work
+ * structure in your own data struct, and use container_of in a
+ * work function to get back to your data struct.
+ */
+struct btrfs_work {
+	/*
+	 * func should be set to the function you want called
+	 * your work struct is passed as the only arg
+	 *
+	 * ordered_func must be set for work sent to an ordered work queue,
+	 * and it is called to complete a given work item in the same
+	 * order they were sent to the queue.
+	 */
+	void (*func)(struct btrfs_work *work);
+	void (*ordered_func)(struct btrfs_work *work);
+	void (*ordered_free)(struct btrfs_work *work);
+
+	/*
+	 * flags should be set to zero.  It is used to make sure the
+	 * struct is only inserted once into the list.
+	 */
+	unsigned long flags;
+
+	/* don't touch these */
+	struct btrfs_worker_thread *worker;
+	struct list_head list;
+	struct list_head order_list;
+};
+
+struct btrfs_workers {
+	/* current number of running workers */
+	int num_workers;
+
+	/* max number of workers allowed.  changed by btrfs_start_workers */
+	int max_workers;
+
+	/* once a worker has this many requests or fewer, it is idle */
+	int idle_thresh;
+
+	/* force completions in the order they were queued */
+	int ordered;
+
+	/* list with all the work threads.  The workers on the idle thread
+	 * may be actively servicing jobs, but they haven't yet hit the
+	 * idle thresh limit above.
+	 */
+	struct list_head worker_list;
+	struct list_head idle_list;
+
+	/*
+	 * when operating in ordered mode, this maintains the list
+	 * of work items waiting for completion
+	 */
+	struct list_head order_list;
+
+	/* lock for finding the next worker thread to queue on */
+	spinlock_t lock;
+
+	/* extra name for this worker, used for current->name */
+	char *name;
+};
+
+int btrfs_queue_worker(struct btrfs_workers *workers, struct btrfs_work *work);
+int btrfs_start_workers(struct btrfs_workers *workers, int num_workers);
+int btrfs_stop_workers(struct btrfs_workers *workers);
+void btrfs_init_workers(struct btrfs_workers *workers, char *name, int max);
+int btrfs_requeue_work(struct btrfs_work *work);
+#endif
-- 
1.6.0.2