From: Tang Yizhou <yizhou.tang@shopee.com>

Now when I/O is submitted to a partition, the per-CPU in_flight[]
counter is incremented only on the partition's block_device, not on the
underlying whole disk. This leads to a problem which can be shown by a
fio test:

lsblk
  NAME     MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
  mydev    252:1    0   20G  0 disk
  └─mydev1 259:0    0   10G  0 part

iostat -xp 1
  Device       r/s        rkB/s      ... aqu-sz   %util
  mydev    128153.00  512612.00      ...  13.22   72.20
  mydev1   128154.00  512616.00      ...  13.22  100.00

%util is different between mydev and mydev1, which is unexpected.

This is the cumulative effect of a series of patches. The root cause is
commit e016b78201a2 ("block: return just one value from part_in_flight"),
which deleted the branch in part_in_flight() that aggregated the whole-disk
in_flight count on top of the partition's. Then the second commit is
commit 10ec5e86f9b8 ("block: merge part_{inc,dev}_in_flight into their
only callers"), which folded the whole-disk in_flight accounting into
generic_start_io_acct() and generic_end_io_acct(). Those two helpers
were then removed by commit e722fff238bb ("block: remove
generic_{start,end}_io_acct"), and from that point on the whole disk's
in_flight is no longer accounted at all.

In update_io_ticks(), if calling bdev_count_inflight() finds that the
inflight value of the whole device is 0, the accumulation of io_ticks will
be skipped, causing the reported util% value to be underestimated.

Fix it by restoring the whole-disk in_flight accounting.

Fixes: e016b78201a2 ("block: return just one value from part_in_flight")

Suggested-by: Leon Hwang <leon.huangfu@shopee.com>
Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
---
v2: Update commit message.
 block/blk-core.c | 4 ++++
 block/blk-mq.c   | 6 ++++++
 2 files changed, 10 insertions(+)

diff --git a/block/blk-core.c b/block/blk-core.c
index 17450058ea6d..03f4b7015e69 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1043,6 +1043,8 @@ unsigned long bdev_start_io_acct(struct block_device *bdev, enum req_op op,
 	part_stat_lock();
 	update_io_ticks(bdev, start_time, false);
 	part_stat_local_inc(bdev, in_flight[op_is_write(op)]);
+	if (bdev_is_partition(bdev))
+		part_stat_local_inc(bdev_whole(bdev), in_flight[op_is_write(op)]);
 	part_stat_unlock();
 
 	return start_time;
@@ -1074,6 +1076,8 @@ void bdev_end_io_acct(struct block_device *bdev, enum req_op op,
 	part_stat_add(bdev, sectors[sgrp], sectors);
 	part_stat_add(bdev, nsecs[sgrp], jiffies_to_nsecs(duration));
 	part_stat_local_dec(bdev, in_flight[op_is_write(op)]);
+	if (bdev_is_partition(bdev))
+		part_stat_local_dec(bdev_whole(bdev), in_flight[op_is_write(op)]);
 	part_stat_unlock();
 }
 EXPORT_SYMBOL(bdev_end_io_acct);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index d0c37daf568f..60ead16f1496 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1084,6 +1084,9 @@ static inline void blk_account_io_done(struct request *req, u64 now)
 		part_stat_add(req->part, nsecs[sgrp], now - req->start_time_ns);
 		part_stat_local_dec(req->part,
 				    in_flight[op_is_write(req_op(req))]);
+		if (bdev_is_partition(req->part))
+			part_stat_local_dec(bdev_whole(req->part),
+					    in_flight[op_is_write(req_op(req))]);
 		part_stat_unlock();
 	}
 }
@@ -1144,6 +1147,9 @@ static inline void blk_account_io_start(struct request *req)
 	part_stat_lock();
 	update_io_ticks(req->part, jiffies, false);
 	part_stat_local_inc(req->part, in_flight[op_is_write(req_op(req))]);
+	if (bdev_is_partition(req->part))
+		part_stat_local_inc(bdev_whole(req->part),
+				    in_flight[op_is_write(req_op(req))]);
 	part_stat_unlock();
 }
 
-- 
2.43.0

This looks unchanged from v1, did you just resend that?

Sorry I sent this before reading your email. Please refer to patch v3.

Best regards,
Yi