block: add sysfs knob for turning off disk entropy contributions

There are two reasons for doing this:

- On SSD disks, the completion times aren't as random as they
  are for rotational drives. So it's questionable whether they
  should contribute to the random pool in the first place.

- Calling add_disk_randomness() has a lot of overhead.

This adds /sys/block/<dev>/queue/add_random that will allow you to
switch off on a per-device basis. The default setting is on, so there
should be no functional changes from this patch.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/block/blk-core.c b/block/blk-core.c
index f0640d7..b4131d2 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2111,7 +2111,8 @@ static bool blk_update_bidi_request(struct request *rq, int error,
 	    blk_update_request(rq->next_rq, error, bidi_bytes))
 		return true;
 
-	add_disk_randomness(rq->rq_disk);
+	if (blk_queue_add_random(rq->q))
+		add_disk_randomness(rq->rq_disk);
 
 	return false;
 }