Re: request to revert libata-convert-to-block-tagging patches

[Jens Axboe <jens.axboe@oracle.com>]

On Mon, Nov 10 2008, Jens Axboe wrote:
> > 2. blk-tag offsets allocation for non-sync requests.  I'm not confident
> > this is safe.  Till now, if there was only single command in flight for
> > the port, it was guaranteed that the qc gets tag zero whether the device
> > is NCQ capable or not.  qc allocation is tied tightly with hardware
> > command slot allocation and I don't think it's wise to change this
> > assumption.
> > 
> > #1 is easy to fix but #2 requires either adding a spinlock or two atomic
> > variables to struct blk_queue_tag to keep the current behavior while
> > guaranteeing that tags are used in order.  Also, there's delay between
> > libata marks a request complete and the request actually gets completed
> > and the tag is freed.  If another request gets issued inbetween, the tag
> > number can't be guaranteed.  This can be worked around by re-mapping tag
> > number in libata depending on command type but, well then, it's worse
> > than the original implementation.
> 
> Or we could just change the blk-tag.c logic to stop of
> find_first_zero_bit() returns >= some_value instead of starting at an
> offset? You don't need any extra locking for that.

Something like the below.

diff --git a/block/blk-tag.c b/block/blk-tag.c
index c0d419e..451e7ce 100644
--- a/block/blk-tag.c
+++ b/block/blk-tag.c
@@ -337,7 +337,7 @@ EXPORT_SYMBOL(blk_queue_end_tag);
 int blk_queue_start_tag(struct request_queue *q, struct request *rq)
 {
 	struct blk_queue_tag *bqt = q->queue_tags;
-	unsigned max_depth, offset;
+	unsigned max_depth;
 	int tag;
 
 	if (unlikely((rq->cmd_flags & REQ_QUEUED))) {
@@ -356,13 +356,11 @@ int blk_queue_start_tag(struct request_queue *q, struct request *rq)
 	 * to starve sync IO on behalf of flooding async IO.
 	 */
 	max_depth = bqt->max_depth;
-	if (rq_is_sync(rq))
-		offset = 0;
-	else
-		offset = max_depth >> 2;
+	if (!rq_is_sync(rq))
+		max_depth = 3 * max_depth / 4;
 
 	do {
-		tag = find_next_zero_bit(bqt->tag_map, max_depth, offset);
+		tag = find_first_zero_bit(bqt->tag_map, max_depth);
 		if (tag >= max_depth)
 			return 1;
 

-- 
Jens Axboe