block,bfq: Disable writeback throttling

Similarly to CFQ, BFQ has its write-throttling heuristics, and it
is better not to combine them with further write-throttling
heuristics of a different nature.
So this commit disables write-back throttling for a device if BFQ
is used as I/O scheduler for that device.

Signed-off-by: Luca Miccio <lucmiccio@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Tested-by: Lee Tibbert <lee.tibbert@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -108,6 +108,7 @@
 #include "blk-mq-tag.h"
 #include "blk-mq-sched.h"
 #include "bfq-iosched.h"
+#include "blk-wbt.h"
 
 #define BFQ_BFQQ_FNS(name)						\
 void bfq_mark_bfqq_##name(struct bfq_queue *bfqq)			\
@@ -4810,7 +4811,7 @@ static int bfq_init_queue(struct request_queue *q, struct elevator_type *e)
 	bfq_init_root_group(bfqd->root_group, bfqd);
 	bfq_init_entity(&bfqd->oom_bfqq.entity, bfqd->root_group);
 
-
+	wbt_disable_default(q);
 	return 0;
 
 out_free:

block/blk-wbt.c

@@ -654,7 +654,7 @@ void wbt_set_write_cache(struct rq_wb *rwb, bool write_cache_on)
 }
 
 /*
- * Disable wbt, if enabled by default. Only called from CFQ.
+ * Disable wbt, if enabled by default.
  */
 void wbt_disable_default(struct request_queue *q)
 {