[Devel] [PATCH VZ10 01/11] drivers/md/dm-rq: fix time accounting for request-based dm drivers

[Andrey Zhadchenko]

Currently all requests for request-based dm drivers are falling in
the last bin for both normal and precise counting due to incorrect
calculations.

Ordinary counting expects that start_time passed to dm_stats_account_io()
is an absolute value and substracts it from jiffies to get request time.
However rq_end_stats() calculates and passes request duration. So we
actually get jiffies - <request_duration_in_jiffies>, which is incorrect.
rq_end_stats() should not do the substraction.

For precise calulation stats->aux_duration_ns is used. It is calculated
in dm_stats_account_io() like that:
stats_aux->duration_ns = ktime_to_ns(ktime_get()) - stats_aux->duration_ns;
The problem is that stats_aux->duration_ns() is set by
dm_stats_record_start() which is called only for bio-based targets.
It should also be called in dm_start_request().

https://virtuozzo.atlassian.net/browse/VSTOR-103846
Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
---
 drivers/md/dm-rq.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c
index 2177666c64f32..58ad4c94d6119 100644
--- a/drivers/md/dm-rq.c
+++ b/drivers/md/dm-rq.c
@@ -129,7 +129,6 @@ static void rq_end_stats(struct mapped_device *md, struct request *orig)
 	if (unlikely(dm_stats_used(&md->stats))) {
 		struct dm_rq_target_io *tio = tio_from_request(orig);
 
-		tio->duration_jiffies = jiffies - tio->duration_jiffies;
 		dm_stats_account_io(&md->stats, rq_data_dir(orig),
 				    blk_rq_pos(orig), tio->n_sectors, true,
 				    tio->duration_jiffies, &tio->stats_aux);
@@ -448,6 +447,8 @@ static void dm_start_request(struct mapped_device *md, struct request *orig)
 
 		tio->duration_jiffies = jiffies;
 		tio->n_sectors = blk_rq_sectors(orig);
+
+		dm_stats_record_start(&md->stats, &tio->stats_aux);
 		dm_stats_account_io(&md->stats, rq_data_dir(orig),
 				    blk_rq_pos(orig), tio->n_sectors, false, 0,
 				    &tio->stats_aux);
-- 
2.43.5