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, 4 insertions, 1 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 09a840264d6f..b8224ea4a5de 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -467,11 +467,13 @@ struct request_queue
 #define QUEUE_FLAG_IO_STAT     15       /* do IO stats */
 #define QUEUE_FLAG_DISCARD     16       /* supports DISCARD */
 #define QUEUE_FLAG_NOXMERGES   17       /* No extended merges */
+#define QUEUE_FLAG_ADD_RANDOM  18       /* Contributes to random pool */
 
 #define QUEUE_FLAG_DEFAULT      ((1 << QUEUE_FLAG_IO_STAT) |            \
                                  (1 << QUEUE_FLAG_CLUSTER) |            \
                                  (1 << QUEUE_FLAG_STACKABLE)    |       \
-                                 (1 << QUEUE_FLAG_SAME_COMP))
+                                 (1 << QUEUE_FLAG_SAME_COMP)    |       \
+                                 (1 << QUEUE_FLAG_ADD_RANDOM))
 
 static inline int queue_is_locked(struct request_queue *q)
 {
@@ -596,6 +598,7 @@ enum {
         test_bit(QUEUE_FLAG_NOXMERGES, &(q)->queue_flags)
 #define blk_queue_nonrot(q)     test_bit(QUEUE_FLAG_NONROT, &(q)->queue_flags)
 #define blk_queue_io_stat(q)    test_bit(QUEUE_FLAG_IO_STAT, &(q)->queue_flags)
+#define blk_queue_add_random(q) test_bit(QUEUE_FLAG_ADD_RANDOM, &(q)->queue_flags)
 #define blk_queue_flushing(q)   ((q)->ordseq)
 #define blk_queue_stackable(q)  \
         test_bit(QUEUE_FLAG_STACKABLE, &(q)->queue_flags)