Doing high IOPS testing with blk-cgroups enabled spends ~15-20% of the
time just doing ktime_get_ns() -> readtsc. We essentially read and
set the start time twice, one for the bio and then again when that bio
is mapped to a request.
Given that the time between the two is very short, inherit the bio
start time instead of reading it again. This cuts 1/3rd of the overhead
of the time keeping.
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
block/blk-mq.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index a8c437afc2c3..a40c94505680 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -718,7 +718,14 @@ void blk_mq_start_request(struct request *rq)
trace_block_rq_issue(rq);
if (test_bit(QUEUE_FLAG_STATS, &q->queue_flags)) {
- rq->io_start_time_ns = ktime_get_ns();
+ u64 start_time;
+#ifdef CONFIG_BLK_CGROUP
+ if (rq->bio)
+ start_time = bio_issue_time(&rq->bio->bi_issue);
+ else
+#endif
+ start_time = ktime_get_ns();
+ rq->io_start_time_ns = start_time;
rq->stats_sectors = blk_rq_sectors(rq);
rq->rq_flags |= RQF_STATS;
rq_qos_issue(q, rq);
--
2.33.0
