block: add queue flag for disabling SG merging

If devices are not SG starved, we waste a lot of time potentially
collapsing SG segments. Enough that 1.5% of the CPU time goes
to this, at only 400K IOPS. Add a queue flag, QUEUE_FLAG_NO_SG_MERGE,
which just returns the number of vectors in a bio instead of looping
over all segments and checking for collapsible ones.

Add a BLK_MQ_F_SG_MERGE flag so that drivers can opt-in on the sg
merging, if they so desire.

Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 098304576d51..695b9fd41efe 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -510,6 +510,7 @@ struct request_queue {
 #define QUEUE_FLAG_SAME_FORCE  18       /* force complete on same CPU */
 #define QUEUE_FLAG_DEAD        19       /* queue tear-down finished */
 #define QUEUE_FLAG_INIT_DONE   20       /* queue is initialized */
+#define QUEUE_FLAG_NO_SG_MERGE 21       /* don't attempt to merge SG segments*/
 
 #define QUEUE_FLAG_DEFAULT      ((1 << QUEUE_FLAG_IO_STAT) |            \
                                  (1 << QUEUE_FLAG_STACKABLE)    |       \