[PATCH] standalone elevator noop

From: Nick Piggin <piggin@cyberone.com.au>

Following patch seperates elevator noop, and allows it to be treated
like the other schedulers.

drivers/block/Kconfig.iosched

+config IOSCHED_NOOP
+	bool "No-op I/O scheduler" if EMBEDDED
+	default y
+	---help---
+	  The no-op I/O scheduler is a minimal scheduler that does basic merging
+	  and sorting. Its main uses include non-disk based block devices like
+	  memory devices, and specialised software or hardware environments
+	  that do their own scheduling and require only minimal assistance from
+	  the kernel.
+
 config IOSCHED_AS
 	bool "Anticipatory I/O scheduler" if EMBEDDED
 	default y
+	---help---
+	  The anticipatory I/O scheduler is the default disk scheduler. It is
+	  generally a good choice for most environments, but is quite large and
+	  complex when compared to the deadline I/O scheduler, it can also be
+	  slower in some cases especially some database loads.
 
 config IOSCHED_DEADLINE
 	bool "Deadline I/O scheduler" if EMBEDDED
 	default y
+	---help---
+	  The deadline I/O scheduler is simple and compact, and is often as
+	  good as the anticipatory I/O scheduler, and in some database
+	  workloads, better. In the case of a single process performing I/O to
+	  a disk at any one time, its behaviour is almost identical to the
+	  anticipatory I/O scheduler and so is a good choice.
 

drivers/block/Makefile

@@ -15,6 +15,7 @@
 
 obj-y	:= elevator.o ll_rw_blk.o ioctl.o genhd.o scsi_ioctl.o
 
+obj-$(CONFIG_IOSCHED_NOOP)	+= noop-iosched.o
 obj-$(CONFIG_IOSCHED_AS)	+= as-iosched.o
 obj-$(CONFIG_IOSCHED_DEADLINE)	+= deadline-iosched.o
 obj-$(CONFIG_MAC_FLOPPY)	+= swim3.o

drivers/block/elevator.c

@@ -86,72 +86,6 @@ inline int elv_try_last_merge(request_queue_t *q, struct bio *bio)
 	return ELEVATOR_NO_MERGE;
 }
 
-/*
- * elevator noop
- *
- * See if we can find a request that this buffer can be coalesced with.
- */
-int elevator_noop_merge(request_queue_t *q, struct list_head **insert,
-			struct bio *bio)
-{
-	struct list_head *entry = &q->queue_head;
-	struct request *__rq;
-	int ret;
-
-	if ((ret = elv_try_last_merge(q, bio))) {
-		*insert = q->last_merge;
-		return ret;
-	}
-
-	while ((entry = entry->prev) != &q->queue_head) {
-		__rq = list_entry_rq(entry);
-
-		if (__rq->flags & (REQ_SOFTBARRIER | REQ_HARDBARRIER))
-			break;
-		else if (__rq->flags & REQ_STARTED)
-			break;
-
-		if (!blk_fs_request(__rq))
-			continue;
-
-		if ((ret = elv_try_merge(__rq, bio))) {
-			*insert = &__rq->queuelist;
-			q->last_merge = &__rq->queuelist;
-			return ret;
-		}
-	}
-
-	return ELEVATOR_NO_MERGE;
-}
-
-void elevator_noop_merge_requests(request_queue_t *q, struct request *req,
-				  struct request *next)
-{
-	list_del_init(&next->queuelist);
-}
-
-void elevator_noop_add_request(request_queue_t *q, struct request *rq,
-			       struct list_head *insert_here)
-{
-	list_add_tail(&rq->queuelist, &q->queue_head);
-
-	/*
-	 * new merges must not precede this barrier
-	 */
-	if (rq->flags & REQ_HARDBARRIER)
-		q->last_merge = NULL;
-	else if (!q->last_merge)
-		q->last_merge = &rq->queuelist;
-}
-
-struct request *elevator_noop_next_request(request_queue_t *q)
-{
-	if (!list_empty(&q->queue_head))
-		return list_entry_rq(q->queue_head.next);
-
-	return NULL;
-}
-
 /*
  * general block -> elevator interface starts here
  */
@@ -415,18 +349,8 @@ void elv_unregister_queue(struct request_queue *q)
 	}
 }
 
-elevator_t elevator_noop = {
-	.elevator_merge_fn		= elevator_noop_merge,
-	.elevator_merge_req_fn		= elevator_noop_merge_requests,
-	.elevator_next_req_fn		= elevator_noop_next_request,
-	.elevator_add_req_fn		= elevator_noop_add_request,
-	.elevator_name			= "noop",
-};
-
 module_init(elevator_global_init);
 
-EXPORT_SYMBOL(elevator_noop);
-
 EXPORT_SYMBOL(elv_add_request);
 EXPORT_SYMBOL(__elv_add_request);
 EXPORT_SYMBOL(elv_requeue_request);

drivers/block/ll_rw_blk.c

@@ -1232,11 +1232,14 @@ static elevator_t *chosen_elevator =
 	&iosched_as;
 #elif defined(CONFIG_IOSCHED_DEADLINE)
 	&iosched_deadline;
-#else
+#elif defined(CONFIG_IOSCHED_NOOP)
 	&elevator_noop;
+#else
+	NULL;
+#error "You must have at least 1 I/O scheduler selected"
 #endif
 
-#if defined(CONFIG_IOSCHED_AS) || defined(CONFIG_IOSCHED_DEADLINE)
+#if defined(CONFIG_IOSCHED_AS) || defined(CONFIG_IOSCHED_DEADLINE) || defined (CONFIG_IOSCHED_NOOP)
 static int __init elevator_setup(char *str)
 {
 #ifdef CONFIG_IOSCHED_DEADLINE
@@ -1246,12 +1249,16 @@ static int __init elevator_setup(char *str)
 #ifdef CONFIG_IOSCHED_AS
 	if (!strcmp(str, "as"))
 		chosen_elevator = &iosched_as;
+#endif
+#ifdef CONFIG_IOSCHED_NOOP
+	if (!strcmp(str, "noop"))
+		chosen_elevator = &elevator_noop;
 #endif
 	return 1;
 }
 
 __setup("elevator=", elevator_setup);
-#endif /* CONFIG_IOSCHED_AS || CONFIG_IOSCHED_DEADLINE */
+#endif /* CONFIG_IOSCHED_AS || CONFIG_IOSCHED_DEADLINE || CONFIG_IOSCHED_NOOP */
 
 request_queue_t *blk_alloc_queue(int gfp_mask)
 {

drivers/block/noop-iosched.c

+/*
+ * elevator noop
+ */
+#include <linux/kernel.h>
+#include <linux/fs.h>
+#include <linux/blkdev.h>
+#include <linux/elevator.h>
+#include <linux/bio.h>
+#include <linux/config.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/init.h>
+#include <linux/compiler.h>
+
+#include <asm/uaccess.h>
+
+/*
+ * See if we can find a request that this buffer can be coalesced with.
+ */
+int elevator_noop_merge(request_queue_t *q, struct list_head **insert,
+			struct bio *bio)
+{
+	struct list_head *entry = &q->queue_head;
+	struct request *__rq;
+	int ret;
+
+	if ((ret = elv_try_last_merge(q, bio))) {
+		*insert = q->last_merge;
+		return ret;
+	}
+
+	while ((entry = entry->prev) != &q->queue_head) {
+		__rq = list_entry_rq(entry);
+
+		if (__rq->flags & (REQ_SOFTBARRIER | REQ_HARDBARRIER))
+			break;
+		else if (__rq->flags & REQ_STARTED)
+			break;
+
+		if (!blk_fs_request(__rq))
+			continue;
+
+		if ((ret = elv_try_merge(__rq, bio))) {
+			*insert = &__rq->queuelist;
+			q->last_merge = &__rq->queuelist;
+			return ret;
+		}
+	}
+
+	return ELEVATOR_NO_MERGE;
+}
+
+void elevator_noop_merge_requests(request_queue_t *q, struct request *req,
+				  struct request *next)
+{
+	list_del_init(&next->queuelist);
+}
+
+void elevator_noop_add_request(request_queue_t *q, struct request *rq,
+			       struct list_head *insert_here)
+{
+	list_add_tail(&rq->queuelist, &q->queue_head);
+
+	/*
+	 * new merges must not precede this barrier
+	 */
+	if (rq->flags & REQ_HARDBARRIER)
+		q->last_merge = NULL;
+	else if (!q->last_merge)
+		q->last_merge = &rq->queuelist;
+}
+
+struct request *elevator_noop_next_request(request_queue_t *q)
+{
+	if (!list_empty(&q->queue_head))
+		return list_entry_rq(q->queue_head.next);
+
+	return NULL;
+}
+
+elevator_t elevator_noop = {
+	.elevator_merge_fn		= elevator_noop_merge,
+	.elevator_merge_req_fn		= elevator_noop_merge_requests,
+	.elevator_next_req_fn		= elevator_noop_next_request,
+	.elevator_add_req_fn		= elevator_noop_add_request,
+	.elevator_name			= "noop",
+};
+
+EXPORT_SYMBOL(elevator_noop);