Create a dynamically sized pool of threads for doing very slow work items

Create a dynamically sized pool of threads for doing very slow work items, such
as invoking mkdir() or rmdir() - things that may take a long time and may
sleep, holding mutexes/semaphores and hogging a thread, and are thus unsuitable
for workqueues.

The number of threads is always at least a settable minimum, but more are
started when there's more work to do, up to a limit.  Because of the nature of
the load, it's not suitable for a 1-thread-per-CPU type pool.  A system with
one CPU may well want several threads.

This is used by FS-Cache to do slow caching operations in the background, such
as looking up, creating or deleting cache objects.
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Daire Byrne <Daire.Byrne@framestore.com>

include/linux/slow-work.h

+/* Worker thread pool for slow items, such as filesystem lookups or mkdirs
+ *
+ * Copyright (C) 2008 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public Licence
+ * as published by the Free Software Foundation; either version
+ * 2 of the Licence, or (at your option) any later version.
+ */
+
+#ifndef _LINUX_SLOW_WORK_H
+#define _LINUX_SLOW_WORK_H
+
+#ifdef CONFIG_SLOW_WORK
+
+struct slow_work;
+
+/*
+ * The operations used to support slow work items
+ */
+struct slow_work_ops {
+	/* get a ref on a work item
+	 * - return 0 if successful, -ve if not
+	 */
+	int (*get_ref)(struct slow_work *work);
+
+	/* discard a ref to a work item */
+	void (*put_ref)(struct slow_work *work);
+
+	/* execute a work item */
+	void (*execute)(struct slow_work *work);
+};
+
+/*
+ * A slow work item
+ * - A reference is held on the parent object by the thread pool when it is
+ *   queued
+ */
+struct slow_work {
+	unsigned long		flags;
+#define SLOW_WORK_PENDING	0	/* item pending (further) execution */
+#define SLOW_WORK_EXECUTING	1	/* item currently executing */
+#define SLOW_WORK_ENQ_DEFERRED	2	/* item enqueue deferred */
+#define SLOW_WORK_VERY_SLOW	3	/* item is very slow */
+	const struct slow_work_ops *ops; /* operations table for this item */
+	struct list_head	link;	/* link in queue */
+};
+
+/**
+ * slow_work_init - Initialise a slow work item
+ * @work: The work item to initialise
+ * @ops: The operations to use to handle the slow work item
+ *
+ * Initialise a slow work item.
+ */
+static inline void slow_work_init(struct slow_work *work,
+				  const struct slow_work_ops *ops)
+{
+	work->flags = 0;
+	work->ops = ops;
+	INIT_LIST_HEAD(&work->link);
+}
+
+/**
+ * slow_work_init - Initialise a very slow work item
+ * @work: The work item to initialise
+ * @ops: The operations to use to handle the slow work item
+ *
+ * Initialise a very slow work item.  This item will be restricted such that
+ * only a certain number of the pool threads will be able to execute items of
+ * this type.
+ */
+static inline void vslow_work_init(struct slow_work *work,
+				   const struct slow_work_ops *ops)
+{
+	work->flags = 1 << SLOW_WORK_VERY_SLOW;
+	work->ops = ops;
+	INIT_LIST_HEAD(&work->link);
+}
+
+extern int slow_work_enqueue(struct slow_work *work);
+extern int slow_work_register_user(void);
+extern void slow_work_unregister_user(void);
+
+
+#endif /* CONFIG_SLOW_WORK */
+#endif /* _LINUX_SLOW_WORK_H */

init/Kconfig

@@ -1014,6 +1014,18 @@ config MARKERS
 
 source "arch/Kconfig"
 
+config SLOW_WORK
+	default n
+	bool "Enable slow work thread pool"
+	help
+	  The slow work thread pool provides a number of dynamically allocated
+	  threads that can be used by the kernel to perform operations that
+	  take a relatively long time.
+
+	  An example of this would be CacheFiles doing a path lookup followed
+	  by a series of mkdirs and a create call, all of which have to touch
+	  disk.
+
 endmenu		# General setup
 
 config HAVE_GENERIC_DMA_COHERENT

kernel/Makefile

@@ -93,6 +93,7 @@ obj-$(CONFIG_HAVE_GENERIC_DMA_COHERENT) += dma-coherent.o
 obj-$(CONFIG_FUNCTION_TRACER) += trace/
 obj-$(CONFIG_TRACING) += trace/
 obj-$(CONFIG_SMP) += sched_cpupri.o
+obj-$(CONFIG_SLOW_WORK) += slow-work.o
 
 ifneq ($(CONFIG_SCHED_OMIT_FRAME_POINTER),y)
 # According to Alan Modra <alan@linuxcare.com.au>, the -fno-omit-frame-pointer is