| Message ID | YmmeOLfo5lzc+8yI@slm.duckdns.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [block-5.18] Revert "block: inherit request start time from bio for BLK_CGROUP" | expand |
On Wed, 27 Apr 2022 09:49:12 -1000, Tejun Heo wrote: > 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. > > [...] Applied, thanks! [1/1] Revert "block: inherit request start time from bio for BLK_CGROUP" commit: 4cddeacad6d4b23493a108d0705e7d2ab89ba5a3 Best regards,
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);
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+ --- block/blk-mq.c | 9 +-------- 1 file changed, 1 insertion(+), 8 deletions(-)