Btrfs: Add async worker threads for pre and post IO checksumming

Btrfs has been using workqueues to spread the checksumming load across
other CPUs in the system.  But, workqueues only schedule work on the
same CPU that queued the work, giving them a limited benefit for systems with
higher CPU counts.

This code adds a generic facility to schedule work with pools of kthreads,
and changes the bio submission code to queue bios up.  The queueing is
important to make sure large numbers of procs on the system don't
turn streaming workloads into random workloads by sending IO down
concurrently.

The end result of all of this is much higher performance (and CPU usage) when
doing checksumming on large machines.  Two worker pools are created,
one for writes and one for endio processing.  The two could deadlock if
we tried to service both from a single pool.

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

1 files changed, 78 insertions, 0 deletions

diff --git a/fs/btrfs/async-thread.h b/fs/btrfs/async-thread.h
new file mode 100644
index 000000000000..52fc9da0f9e7
--- /dev/null
+++ b/fs/btrfs/async-thread.h
@@ -0,0 +1,78 @@
+/*
+ * 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 {
+	/*
+	 * only func should be set to the function you want called
+	 * your work struct is passed as the only arg
+	 */
+	void (*func)(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 btrfs_workers {
+	/* current number of running workers */
+	int num_workers;
+
+	/* max number of workers allowed.  changed by btrfs_start_workers */
+	int max_workers;
+
+	/* list with all the work threads */
+	struct list_head worker_list;
+
+	/* the last worker thread to have something queued */
+	struct btrfs_worker_thread *last;
+
+	/* lock for finding the next worker thread to queue on */
+	spinlock_t lock;
+};
+
+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, int max);
+int btrfs_requeue_work(struct btrfs_work *work);
+#endif