linux-kernel - [PATCH block-5.18] Revert "block: inherit request start time from bio for BLK

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Message-ID: <YmmeOLfo5lzc+8yI@slm.duckdns.org>
Date:   Wed, 27 Apr 2022 09:49:12 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, Saravanan D <saravanand@...com>,
        Christopher Obbard <chris.obbard@...labora.com>
Subject: [PATCH block-5.18] 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@...nel.org>
Cc: stable@...r.kernel.org # v5.16+
---
 block/blk-mq.c | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index c4370d2761706..84d749511f551 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);
-- 
2.36.0