path: root/block/blk-settings.c
diff options
authorKiyoshi Ueda <>2008-10-01 16:12:15 +0200
committerJens Axboe <>2008-10-09 08:56:20 +0200
commitef9e3facdf1fe1228721a7c295a76d1b7a0e57ec (patch)
tree33847b1e267895ea58c2399c06cdd23365c7dc73 /block/blk-settings.c
parent336c3d8ce771608815b65bcfa27a17a83b297328 (diff)
block: add lld busy state exporting interface
This patch adds an new interface, blk_lld_busy(), to check lld's busy state from the block layer. blk_lld_busy() calls down into low-level drivers for the checking if the drivers set q->lld_busy_fn() using blk_queue_lld_busy(). This resolves a performance problem on request stacking devices below. Some drivers like scsi mid layer stop dispatching request when they detect busy state on its low-level device like host/target/device. It allows other requests to stay in the I/O scheduler's queue for a chance of merging. Request stacking drivers like request-based dm should follow the same logic. However, there is no generic interface for the stacked device to check if the underlying device(s) are busy. If the request stacking driver dispatches and submits requests to the busy underlying device, the requests will stay in the underlying device's queue without a chance of merging. This causes performance problem on burst I/O load. With this patch, busy state of the underlying device is exported via q->lld_busy_fn(). So the request stacking driver can check it and stop dispatching requests if busy. The underlying device driver must return the busy state appropriately: 1: when the device driver can't process requests immediately. 0: when the device driver can process requests immediately, including abnormal situations where the device driver needs to kill all requests. Signed-off-by: Kiyoshi Ueda <> Signed-off-by: Jun'ichi Nomura <> Cc: Andrew Morton <> Signed-off-by: Jens Axboe <>
Diffstat (limited to 'block/blk-settings.c')
1 files changed, 6 insertions, 0 deletions
diff --git a/block/blk-settings.c b/block/blk-settings.c
index 1d0330d0b40a..b21dcdb64151 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -89,6 +89,12 @@ void blk_queue_rq_timed_out(struct request_queue *q, rq_timed_out_fn *fn)
+void blk_queue_lld_busy(struct request_queue *q, lld_busy_fn *fn)
+ q->lld_busy_fn = fn;
* blk_queue_make_request - define an alternate make_request function for a device
* @q: the request queue for the device to be affected