[PATCH 3/3] block/diskstats: more accurate approximation of io_ticks for slow disks

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

Currently io_ticks is approximated by adding one at each
start and end of requests if jiffies has changed.
This works perfectly for requests shorter than a jiffy.

Fix for slow requests is simple: at the end of request add
count of jiffies passed since last update.

Fixes: 5b18b5a73760 ("block: delete part_round_stats and switch to less precise counting")
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
 block/bio.c           |    8 ++++----
 block/blk-core.c      |    4 ++--
 include/linux/genhd.h |    2 +-
 3 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index c0a60f3e9b7b..245056797999 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1729,14 +1729,14 @@ void bio_check_pages_dirty(struct bio *bio)
 	schedule_work(&bio_dirty_work);
 }
 
-void update_io_ticks(struct hd_struct *part, unsigned long now)
+void update_io_ticks(struct hd_struct *part, unsigned long now, bool end)
 {
 	unsigned long stamp;
 again:
 	stamp = READ_ONCE(part->stamp);
 	if (unlikely(stamp != now)) {
 		if (likely(cmpxchg(&part->stamp, stamp, now) == stamp)) {
-			__part_stat_add(part, io_ticks, 1);
+			__part_stat_add(part, io_ticks, end ? now - stamp : 1);
 		}
 	}
 	if (part->partno) {
@@ -1752,7 +1752,7 @@ void generic_start_io_acct(struct request_queue *q, int op,
 
 	part_stat_lock();
 
-	update_io_ticks(part, jiffies);
+	update_io_ticks(part, jiffies, false);
 	part_stat_inc(part, ios[sgrp]);
 	part_stat_add(part, sectors[sgrp], sectors);
 	part_inc_in_flight(q, part, op_is_write(op));
@@ -1770,7 +1770,7 @@ void generic_end_io_acct(struct request_queue *q, int req_op,
 
 	part_stat_lock();
 
-	update_io_ticks(part, now);
+	update_io_ticks(part, now, true);
 	part_stat_add(part, nsecs[sgrp], jiffies_to_nsecs(duration));
 	part_dec_in_flight(q, part, op_is_write(req_op));
 
diff --git a/block/blk-core.c b/block/blk-core.c
index d89168b167e9..6e8f0b9e7731 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1334,7 +1334,7 @@ void blk_account_io_done(struct request *req, u64 now)
 		part_stat_lock();
 		part = req->part;
 
-		update_io_ticks(part, jiffies);
+		update_io_ticks(part, jiffies, true);
 		part_stat_inc(part, ios[sgrp]);
 		part_stat_add(part, nsecs[sgrp], now - req->start_time_ns);
 		part_dec_in_flight(req->q, part, rq_data_dir(req));
@@ -1375,7 +1375,7 @@ void blk_account_io_start(struct request *rq, bool new_io)
 		rq->part = part;
 	}
 
-	update_io_ticks(part, jiffies);
+	update_io_ticks(part, jiffies, false);
 
 	part_stat_unlock();
 }
diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index 2f5a9ed7e86e..8ece8e02c609 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -410,7 +410,7 @@ static inline void free_part_info(struct hd_struct *part)
 	kfree(part->info);
 }
 
-void update_io_ticks(struct hd_struct *part, unsigned long now);
+void update_io_ticks(struct hd_struct *part, unsigned long now, bool end);
 
 /* block/genhd.c */
 extern void device_add_disk(struct device *parent, struct gendisk *disk,