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authorTejun Heo <tj@kernel.org>2022-04-27 09:49:12 -1000
committerJens Axboe <axboe@kernel.dk>2022-04-27 13:51:28 -0600
commit4cddeacad6d4b23493a108d0705e7d2ab89ba5a3 (patch)
tree4cd48b87f9ee474a66de078d79d2bc1bc5625443
parent8c936f9ea11ec4e35e288810a7503b5c841a355f (diff)
Revert "block: inherit request start time from bio for BLK_CGROUP"
This reverts commit 0006707723233cb2a9a23ca19fc3d0864835704c. It has a couple problems: * bio_issue_time() is stored in bio->bi_issue truncated to 51 bits. This overflows in slightly over 26 days. Setting rq->io_start_time_ns with it means that io duration calculation would yield >26days after 26 days of uptime. This, for example, confuses kyber making it cause high IO latencies. * rq->io_start_time_ns should record the time that the IO is issued to the device so that on-device latency can be measured. However, bio_issue_time() is set before the bio goes through the rq-qos controllers (wbt, iolatency, iocost), so when the bio gets throttled in any of the mechanisms, the measured latencies make no sense - on-device latencies end up higher than request-alloc-to-completion latencies. We'll need a smarter way to avoid calling ktime_get_ns() repeatedly back-to-back. For now, let's revert the commit. Signed-off-by: Tejun Heo <tj@kernel.org> Cc: stable@vger.kernel.org # v5.16+ Link: https://lore.kernel.org/r/YmmeOLfo5lzc+8yI@slm.duckdns.org Signed-off-by: Jens Axboe <axboe@kernel.dk>
-rw-r--r--block/blk-mq.c9
1 files changed, 1 insertions, 8 deletions
diff --git a/block/blk-mq.c b/block/blk-mq.c
index c4370d276170..84d749511f55 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1131,14 +1131,7 @@ void blk_mq_start_request(struct request *rq)
trace_block_rq_issue(rq);
if (test_bit(QUEUE_FLAG_STATS, &q->queue_flags)) {
- 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->io_start_time_ns = ktime_get_ns();
rq->stats_sectors = blk_rq_sectors(rq);
rq->rq_flags |= RQF_STATS;
rq_qos_issue(q, rq);