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 <mahrens@delphix.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7645

diff --git a/config/kernel-elevator-change.m4 b/config/kernel-elevator-change.m4
index ace5aa82d3d353297604fd1d3ce8fd4e4babbd82..eba252579bcc5218ccb762257cc01ad6a183ee1c 100644 (file)
--- a/config/kernel-elevator-change.m4
+++ b/config/kernel-elevator-change.m4
@@ -1,6 +1,6 @@
 dnl #
-dnl # 2.6.36 API change
-dnl # Verify the elevator_change() symbol is available.
+dnl # 2.6.36 API, exported elevator_change() symbol
+dnl # 4.12 API, removed elevator_change() symbol
 dnl #
 AC_DEFUN([ZFS_AC_KERNEL_ELEVATOR_CHANGE], [
        AC_MSG_CHECKING([whether elevator_change() is available])

diff --git a/include/linux/blkdev_compat.h b/include/linux/blkdev_compat.h
index 27f056626626695db5ae41650123a43111ac0f65..c8cdf38ef4fef517114c768d8e5defec263d22f1 100644 (file)
--- a/include/linux/blkdev_compat.h
+++ b/include/linux/blkdev_compat.h
@@ -105,17 +105,6 @@ blk_queue_set_write_cache(struct request_queue *q, bool wc, bool fua)
 #define        blk_fs_request(rq)      ((rq)->cmd_type == REQ_TYPE_FS)
 #endif
 
-/*
- * 2.6.27 API change,
- * The blk_queue_stackable() queue flag was added in 2.6.27 to handle dm
- * stacking drivers.  Prior to this request stacking drivers were detected
- * by checking (q->request_fn == NULL), for earlier kernels we revert to
- * this legacy behavior.
- */
-#ifndef blk_queue_stackable
-#define        blk_queue_stackable(q)  ((q)->request_fn == NULL)
-#endif
-
 /*
  * 2.6.34 API change,
  * The blk_queue_max_hw_sectors() function replaces blk_queue_max_sectors().

diff --git a/module/zfs/vdev_disk.c b/module/zfs/vdev_disk.c
index 6dc0544fc6ad1dfa72ad26c5faf37d455007bdb6..c5708cb2b510346593cd996fd133a655f7a37882 100644 (file)
--- a/module/zfs/vdev_disk.c
+++ b/module/zfs/vdev_disk.c
@@ -168,23 +168,20 @@ 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 " \
@@ -198,9 +195,10 @@ 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("ZFS: Unable to set \"%s\" scheduler for %s (%s): %d\n",
+       if (error) {
+               zfs_dbgmsg("Unable to set \"%s\" scheduler for %s (%s): %d\n",
                    elevator, v->vdev_path, device, error);
+       }
 }
 
 /*