block: reserve some tags just for sync IO

[linux-2.6-omap-h63xx.git] / block / blk-tag.c

diff --git a/block/blk-tag.c b/block/blk-tag.c
index 8a99688eb1b1951e35586fb2861cf6e86917c756..c0d419e84ce7f8518e1246e3ec61b07f5c81efe8 100644 (file)
--- a/block/blk-tag.c
+++ b/block/blk-tag.c
@@ -337,6 +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;
        int tag;
 
        if (unlikely((rq->cmd_flags & REQ_QUEUED))) {
@@ -350,10 +351,19 @@ int blk_queue_start_tag(struct request_queue *q, struct request *rq)
        /*
         * Protect against shared tag maps, as we may not have exclusive
         * access to the tag map.
+        *
+        * We reserve a few tags just for sync IO, since we don't want
+        * 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;
+
        do {
-               tag = find_first_zero_bit(bqt->tag_map, bqt->max_depth);
-               if (tag >= bqt->max_depth)
+               tag = find_next_zero_bit(bqt->tag_map, max_depth, offset);
+               if (tag >= max_depth)
                        return 1;
 
        } while (test_and_set_bit_lock(tag, bqt->tag_map));