blk-wbt: don't show valid wbt_lat_usec in sysfs while wbt is disabled

Currently, if wbt is initialized and then disabled by
wbt_disable_default(), sysfs will still show valid wbt_lat_usec, which
will confuse users that wbt is still enabled.

This patch shows wbt_lat_usec as zero if it's disabled.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reported-and-tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20221019121518.3865235-5-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-sysfs.c

@@ -470,6 +470,9 @@ static ssize_t queue_wb_lat_show(struct request_queue *q, char *page)
 	if (!wbt_rq_qos(q))
 		return -EINVAL;
 
+	if (wbt_disabled(q))
+		return sprintf(page, "0\n");
+
 	return sprintf(page, "%llu\n", div_u64(wbt_get_min_lat(q), 1000));
 }
 

block/blk-wbt.c

@@ -422,6 +422,14 @@ static void wbt_update_limits(struct rq_wb *rwb)
 	rwb_wake_all(rwb);
 }
 
+bool wbt_disabled(struct request_queue *q)
+{
+	struct rq_qos *rqos = wbt_rq_qos(q);
+
+	return !rqos || RQWB(rqos)->enable_state == WBT_STATE_OFF_DEFAULT ||
+	       RQWB(rqos)->enable_state == WBT_STATE_OFF_MANUAL;
+}
+
 u64 wbt_get_min_lat(struct request_queue *q)
 {
 	struct rq_qos *rqos = wbt_rq_qos(q);

block/blk-wbt.h

@@ -96,6 +96,7 @@ void wbt_enable_default(struct request_queue *);
 
 u64 wbt_get_min_lat(struct request_queue *q);
 void wbt_set_min_lat(struct request_queue *q, u64 val);
+bool wbt_disabled(struct request_queue *);
 
 void wbt_set_write_cache(struct request_queue *, bool);
 
@@ -127,6 +128,10 @@ static inline u64 wbt_default_latency_nsec(struct request_queue *q)
 {
 	return 0;
 }
+static inline bool wbt_disabled(struct request_queue *q)
+{
+	return true;
+}
 
 #endif /* CONFIG_BLK_WBT */