Linux 4.14 compat: blk_queue_stackable()

The blk_queue_stackable() function was replaced in the 4.14 kernel
by queue_is_rq_based(), commit torvalds/linux@5fdee212.  This change
resulted in the default elevator being used which can negatively
impact performance.

Rather than adding additional compatibility code to detect the
new interface unconditionally attempt to set the elevator.  Since
we expect this to fail for block devices without an elevator the
error message has been moved in to zfs_dbgmsg().

Finally, it was observed that the elevator_change() was removed
from the 4.12 kernel, commit torvalds/linux@c033269.  Update the
comment to clearly specify which are expected to export the
elevator_change() symbol.

Reviewed-by: Matthew Ahrens <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #7645 

1 files changed, 11 insertions, 14 deletions

diff --git a/module/zfs/vdev_disk.c b/module/zfs/vdev_disk.c
index 11744c1c1..996bab43c 100644
--- a/module/zfs/vdev_disk.c
+++ b/module/zfs/vdev_disk.c
@@ -167,23 +167,20 @@ vdev_elevator_switch(vdev_t *v, char *elevator)
 	if (!v->vdev_wholedisk && strncmp(device, "dm-", 3) != 0)
 		return;
 
-	/* Skip devices without schedulers (loop, ram, dm, etc) */
-	if (!q->elevator || !blk_queue_stackable(q))
-		return;
-
 	/* Leave existing scheduler when set to "none" */
 	if ((strncmp(elevator, "none", 4) == 0) && (strlen(elevator) == 4))
 		return;
 
+	/*
+	 * The elevator_change() function was available in kernels from
+	 * 2.6.36 to 4.11.  When not available fall back to using the user
+	 * mode helper functionality to set the elevator via sysfs.  This
+	 * requires /bin/echo and sysfs to be mounted which may not be true
+	 * early in the boot process.
+	 */
 #ifdef HAVE_ELEVATOR_CHANGE
 	error = elevator_change(q, elevator);
 #else
-	/*
-	 * For pre-2.6.36 kernels elevator_change() is not available.
-	 * Therefore we fall back to using a usermodehelper to echo the
-	 * elevator into sysfs;  This requires /bin/echo and sysfs to be
-	 * mounted which may not be true early in the boot process.
-	 */
 #define	SET_SCHEDULER_CMD \
 	"exec 0</dev/null " \
 	"     1>/sys/block/%s/queue/scheduler " \
@@ -197,10 +194,10 @@ vdev_elevator_switch(vdev_t *v, char *elevator)
 	error = call_usermodehelper(argv[0], argv, envp, UMH_WAIT_PROC);
 	strfree(argv[2]);
 #endif /* HAVE_ELEVATOR_CHANGE */
-	if (error)
-		printk(KERN_NOTICE "ZFS: Unable to set \"%s\" scheduler"
-		    " for %s (%s): %d\n", elevator, v->vdev_path, device,
-		    error);
+	if (error) {
+		zfs_dbgmsg("Unable to set \"%s\" scheduler for %s (%s): %d\n",
+		    elevator, v->vdev_path, device, error);
+	}
 }
 
 /*