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Message-Id: <20201005142112.070155997@linuxfoundation.org>
Date: Mon, 5 Oct 2020 17:26:59 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-kernel@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
stable@...r.kernel.org,
Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...nel.dk>
Subject: [PATCH 5.4 47/57] block/diskstats: more accurate approximation of io_ticks for slow disks
From: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
commit 2b8bd423614c595540eaadcfbc702afe8e155e50 upstream.
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: 5b18b5a73760 ("block: delete part_round_stats and switch to less precise counting")
Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Reviewed-by: Ming Lei <ming.lei@...hat.com>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
From: "Banerjee, Debabrata" <dbanerje@...mai.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
Documentation/admin-guide/iostats.rst | 5 ++++-
block/bio.c | 8 ++++----
block/blk-core.c | 4 ++--
include/linux/genhd.h | 2 +-
4 files changed, 11 insertions(+), 8 deletions(-)
--- a/Documentation/admin-guide/iostats.rst
+++ b/Documentation/admin-guide/iostats.rst
@@ -99,7 +99,7 @@ Field 10 -- # of milliseconds spent doin
Since 5.0 this field counts jiffies when at least one request was
started or completed. If request runs more than 2 jiffies then some
- I/O time will not be accounted unless there are other requests.
+ I/O time might be not accounted in case of concurrent requests.
Field 11 -- weighted # of milliseconds spent doing I/Os
This field is incremented at each I/O start, I/O completion, I/O
@@ -133,6 +133,9 @@ are summed (possibly overflowing the uns
summed to) and the result given to the user. There is no convenient
user interface for accessing the per-CPU counters themselves.
+Since 4.19 request times are measured with nanoseconds precision and
+truncated to milliseconds before showing in this interface.
+
Disks vs Partitions
-------------------
--- a/block/bio.c
+++ b/block/bio.c
@@ -1754,14 +1754,14 @@ defer:
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) {
@@ -1777,7 +1777,7 @@ void generic_start_io_acct(struct reques
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));
@@ -1795,7 +1795,7 @@ void generic_end_io_acct(struct request_
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_stat_add(part, time_in_queue, duration);
part_dec_in_flight(q, part, op_is_write(req_op));
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1334,7 +1334,7 @@ void blk_account_io_done(struct request
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_stat_add(part, time_in_queue, nsecs_to_jiffies64(now - req->start_time_ns));
@@ -1376,7 +1376,7 @@ void blk_account_io_start(struct request
rq->part = part;
}
- update_io_ticks(part, jiffies);
+ update_io_ticks(part, jiffies, false);
part_stat_unlock();
}
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -419,7 +419,7 @@ static inline void free_part_info(struct
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,
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