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>

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