Re: [PATCH v3 1/3] block/diskstats: more accurate approximation of io_ticks for slow disks

Ming Lei
2020-03-24T14:07:25Z

On Tue, Mar 24, 2020 at 09:39:40AM +0300, Konstantin Khlebnikov wrote:
> 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
>
> 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@yandex-team.ru>
> ---
> 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(-)
>
> diff --git a/Documentation/admin-guide/iostats.rst b/Documentation/admin-guide/iostats.rst
> index df5b8345c41d..9b14b0c2c9c4 100644
> --- a/Documentation/admin-guide/iostats.rst
> +++ b/Documentation/admin-guide/iostats.rst
> @@ -100,7 +100,7 @@ Field 10 -- # of milliseconds spent doing I/Os (unsigned int)
>
> 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 (unsigned int)
> This field is incremented at each I/O start, I/O completion, I/O
> @@ -143,6 +143,9 @@ are summed (possibly overflowing the unsigned long variable they are
> 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
> -------------------
>
> diff --git a/block/bio.c b/block/bio.c
> index 0985f3422556..b1053eb7af37 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -1762,14 +1762,14 @@ 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) {
> @@ -1785,7 +1785,7 @@ 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));
> @@ -1803,7 +1803,7 @@ 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_stat_add(part, time_in_queue, 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 abfdcf81a228..4401b30a1751 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1337,7 +1337,7 @@ 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_stat_add(part, time_in_queue, nsecs_to_jiffies64(now - req->start_time_ns));
> @@ -1379,7 +1379,7 @@ 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 d5c75df64bba..f1066f10b062 100644
> --- a/include/linux/genhd.h
> +++ b/include/linux/genhd.h
> @@ -467,7 +467,7 @@ 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,
>

Looks fine:

Reviewed-by: Ming Lei <ming.lei@redhat.com>

Thanks,
Ming