block/diskstats: more accurate approximation of io_ticks for slow disks

mainline inclusion
from mainline-v5.7-rc1
commit 2b8bd423
category: bugfix
bugzilla: 187044, https://gitee.com/openeuler/kernel/issues/I5F2BY
CVE: NA

--------------------------------

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

If disk executes just one request at a time and they are longer than two
jiffies then only first and last jiffies will be accounted.

Fix is simple: at the end of request add up into io_ticks jiffies passed
since last update rather than just one jiffy.

Example: common HDD executes random read 4k requests around 12ms.

fio --name=test --filename=/dev/sdb --rw=randread --direct=1 --runtime=30 &
iostat -x 10 sdb

Note changes of iostat's "%util" 8,43% -> 99,99% before/after patch:

Before:

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sdb               0,00     0,00   82,60    0,00   330,40     0,00     8,00     0,96   12,09   12,09    0,00   1,02   8,43

After:

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sdb               0,00     0,00   82,50    0,00   330,00     0,00     8,00     1,00   12,10   12,10    0,00  12,12  99,99

Now io_ticks does not loose time between start and end of requests, but
for queue-depth > 1 some I/O time between adjacent starts might be lost.

For load estimation "%util" is not as useful as average queue length,
but it clearly shows how often disk queue is completely empty.

Fixes: 5b18b5a7 ("block: delete part_round_stats and switch to less precise counting")
Signed-off-by: NKonstantin Khlebnikov <khlebnikov@yandex-team.ru>
Reviewed-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

Conflict:
	block/bio.c
	block/blk-core.c
	include/linux/genhd.h
Signed-off-by: NZhang Wensheng <zhangwensheng5@huawei.com>
Reviewed-by: NYu Kuai <yukuai3@huawei.com>
Reviewed-by: NJason Yan <yanaijie@huawei.com>
Signed-off-by: NYongqiang Liu <liuyongqiang13@huawei.com>

block/bio.c

@@ -1668,14 +1668,14 @@ void bio_check_pages_dirty(struct bio *bio)
 }
 EXPORT_SYMBOL_GPL(bio_check_pages_dirty);
 
-void update_io_ticks(int cpu, struct hd_struct *part, unsigned long now)
+void update_io_ticks(int cpu, 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(cpu, part, io_ticks, now - stamp);
+			__part_stat_add(cpu, part, io_ticks, end ? now - stamp : 1);
 	}
 	if (part->partno) {
 		part = &part_to_disk(part)->part0;
@@ -1709,7 +1709,7 @@ void generic_end_io_acct(struct request_queue *q, int req_op,
 	if (precise_iostat) {
 		part_round_stats(q, cpu, part);
 	} else {
-		update_io_ticks(cpu, part, now);
+		update_io_ticks(cpu, part, now, true);
 		part_stat_add(cpu, part, time_in_queue, duration);
 	}
 	part_stat_add(cpu, part, nsecs[sgrp], jiffies_to_nsecs(duration));

block/blk-core.c

@@ -2809,7 +2809,7 @@ void blk_account_io_done(struct request *req, u64 now)
 		part = req->part;
 
 		if (!precise_iostat) {
-			update_io_ticks(cpu, part, jiffies);
+			update_io_ticks(cpu, part, jiffies, true);
 			part_stat_add(cpu, part, time_in_queue,
 				nsecs_to_jiffies64(now - req->start_time_ns));
 		} else {

include/linux/genhd.h

@@ -427,7 +427,7 @@ static inline void free_part_info(struct hd_struct *part)
 
 /* block/blk-core.c */
 extern void part_round_stats(struct request_queue *q, int cpu, struct hd_struct *part);
-void update_io_ticks(int cpu, struct hd_struct *part, unsigned long now);
+void update_io_ticks(int cpu, struct hd_struct *part, unsigned long now, bool end);
 
 /* block/genhd.c */
 extern void device_add_disk(struct device *parent, struct gendisk *disk);