[PATCH] blk-iocost: account flush sequence latency only once

[Zhe Liu]

A flush sequence with a DATA phase can complete its original request
twice. Both completions invoke ioc_rqos_done(), causing the request to
contribute two latency samples and adding the same queue wait time
twice. This gives such requests disproportionate weight compared to
ordinary writes, distorting the device-wide latency and queue wait
feedback used to adjust iocost's virtual time rate.

Skip requests marked RQF_FLUSH_SEQ in ioc_rqos_done(). The flag is set
when the request enters the flush machinery and is still set when the
data phase completes; blk_flush_restore_request() clears it before the
final completion, so accounting happens only once, after the whole
sequence finishes. The remaining sample covers the request's lifetime
through any postflush, and its queue wait time is accumulated only
once.

Fixes: 7caa47151ab2e ("blkcg: implement blk-iocost")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Zhe Liu <liuzhe1@xxxxxxxxxx>
---
 block/blk-iocost.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index 2ae40138e041..601029221e6f 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -2868,6 +2868,9 @@ static void ioc_rqos_done(struct rq_qos *rqos, struct request *rq)
 	if (!ioc->enabled || !rq->alloc_time_ns || !rq->start_time_ns)
 		return;
 
+	if (rq->rq_flags & RQF_FLUSH_SEQ)
+		return;
+
 	switch (req_op(rq)) {
 	case REQ_OP_READ:
 		pidx = QOS_RLAT;
-- 
2.25.1