[Devel] [PATCH RHEL10 COMMIT] drivers/md/dm-rq: fix time accounting for request-based dm drivers

[Konstantin Khorenko]

The commit is pushed to "branch-rh10-6.12.0-211.39.1.16.x.vz10-ovz" and will appear at git at bitbucket.org:openvz/vzkernel.git
after rh10-6.12.0-211.39.1.16.1.vz10
------>
commit ce4d085badb939d1d8d1b2cc9a5ebb8a8656f54a
Author: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
Date:   Mon Jul 27 19:32:44 2026 +0300

    drivers/md/dm-rq: fix time accounting for request-based dm drivers
    
    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
    Feature: dm-stats: latency and histogram enhancements
    Fixes: e262f3474152 ("dm stats: add support for request-based DM devices")
    Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
    Reviewed-by: Vasileios Almpanis <vasileios.almpanis at virtuozzo.com>
    Reviewed-by: Pavel Tikhomirov <ptikhomirov 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);