[PATCH 1/2] blk-throtl: make latency= absolute

[Tejun Heo]

Currently, the latency= parameter specifies the extra latency on top
of the estimated baseline latency.  This doesn't seem ideal for the
following two reasons.

1. Sometimes we want to use absolute target latencies, especially as
   the baseline latency estimation isn't always reliable.

2. If we want to set a latency target relative to the estimated
   baseline latency, it makes a lot more sense to express the target
   as a percentage of the baseline (say, 120% of the expected latency)
   rather than the baseline latency + an absolute offset.

This patch makes the existing latency= parameter to be interpreted as
an absolute latency target instead of an offset to the baseline.  The
next patch will add support for relative latency target expressed in
terms of percentage.

While this is a userland visible change, io.low support is still
evoling and highly experimental.  This isn't expected to break any
actual usages.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Shaohua Li <shli@xxxxxxxxxx>
---
 block/blk-throttle.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -2299,8 +2299,7 @@ void blk_throtl_bio_endio(struct bio *bi
 
 		bucket = request_bucket_index(
 			blk_stat_size(&bio->bi_issue_stat));
-		threshold = tg->td->avg_buckets[bucket].latency +
-			tg->latency_target;
+		threshold = tg->latency_target;
 		if (lat > threshold)
 			tg->bad_bio_cnt++;
 		/*