[Xen-devel] [PATCH] make blkfront/blktagp2 respect the elevator=xyz command line option

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

For some workloads, CFQ has better performance than the no-op scheduler
that is forced by the blkfront driver.  The only way to set a different
scheduler is the sysfs interface, because elevator_init is called
unconditionally.  This patch allows one to use "elevator=cfq" as well.

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 implemented by this patch is more logical.

Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 block/elevator.c             |    3 ++-
 drivers/xen/blkfront/vbd.c   |   11 ++++++-----
 drivers/xen/blktap2/device.c |   11 ++++++-----
 3 files changed, 14 insertions(+), 11 deletions(-)

diff --git a/block/elevator.c b/block/elevator.c
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -132,7 +132,8 @@
        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
--- a/drivers/xen/blkfront/vbd.c
+++ b/drivers/xen/blkfront/vbd.c
@@ -208,6 +208,8 @@
        /* XXX: release major if 0 */
 }
 
+extern char chosen_elevator[];
+
 static int
 xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size)
 {
@@ -217,11 +219,10 @@
        if (rq == NULL)
                return -1;
 
-#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,10)
-       elevator_init(rq, "noop");
-#else
-       elevator_init(rq, &elevator_noop);
-#endif
+       /* 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);
diff --git a/drivers/xen/blktap2/device.c b/drivers/xen/blktap2/device.c
--- a/drivers/xen/blktap2/device.c
+++ b/drivers/xen/blktap2/device.c
@@ -1034,6 +1034,8 @@
        return 0;
 }
 
+extern char chosen_elevator[];
+
 int
 blktap_device_create(struct blktap *tap)
 {
@@ -1078,11 +1080,10 @@
        if (!rq)
                goto error;
 
-#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,10)
-       elevator_init(rq, "noop");
-#else
-       elevator_init(rq, &elevator_noop);
-#endif
+       /* 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");
 
        gd->queue     = rq;
        rq->queuedata = dev;

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