From: Paolo Bonzini <pbonzini@redhat.com>
Date: Mon, 12 Oct 2009 10:58:37 +0200
Subject: [block] blkfront: respect elevator=xyz cmd line option
Message-id: 1255337917-14250-1-git-send-email-pbonzini@redhat.com
O-Subject: [RHEL 5.5 PATCH v2] blkfront: always respect the elevator=xyz command line option
Bugzilla: 498461
RH-Acked-by: Chris Lalancette <clalance@redhat.com>
RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>
RH-Acked-by: Rik van Riel <riel@redhat.com>
RH-Acked-by: Jeff Moyer <jmoyer@redhat.com>

Bugzilla: 498461

Upstream: posted, but they will remove the elevator_init call altogether

Brew build: http://brewweb.devel.redhat.com/brew/taskinfo?taskID=2023994

In the Bugzilla, the customer noticed that CFQ has better performance
in their environment than the no-op scheduler that is forced by the
blkfront driver.  The customer then asked for us to allow the I/O
scheduler to be changed on the kernel commandline, using "elevator=cfq".

While one could argue that the driver's behavior is expected (after all
"elevator=cfq" is the default and should not have any effect), the
do-what-I-mean behavior requested by the customer is logical and this
is what the patch implements.

The patch does not change the default scheduler.  It just makes the
existing knob affect blkfront.

Cc: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/block/elevator.c b/block/elevator.c
index d56ceae..1e85609 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -133,7 +133,8 @@ static void elevator_attach(request_queue_t *q, struct elevator_queue *eq,
 	eq->elevator_data = data;
 }
 
-static char chosen_elevator[16];
+char chosen_elevator[16];
+EXPORT_SYMBOL_GPL(chosen_elevator);
 
 static int __init elevator_setup(char *str)
 {
diff --git a/drivers/xen/blkfront/vbd.c b/drivers/xen/blkfront/vbd.c
index 36d8d0f..a1f1bff 100644
--- a/drivers/xen/blkfront/vbd.c
+++ b/drivers/xen/blkfront/vbd.c
@@ -207,6 +207,8 @@ xlbd_put_major_info(struct xlbd_major_info *mi)
 	/* XXX: release major if 0 */
 }
 
+extern char chosen_elevator[];
+
 static int
 xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size)
 {
@@ -216,7 +218,10 @@ xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size)
 	if (rq == NULL)
 		return -1;
 
-	elevator_init(rq, "noop");
+	/* Always respect the user's explicitly chosen elevator, but otherwise
+	   pick a different default than CONFIG_DEFAULT_IOSCHED.  */
+	if (!*chosen_elevator)
+		elevator_init(rq, "noop");
 
 	/* Hard sector size and max sectors impersonate the equiv. hardware. */
 	blk_queue_hardsect_size(rq, sector_size);