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

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

On 25/03/2020 06.40, Ming Lei wrote:
> On Tue, Mar 24, 2020 at 10:06:56PM +0800, Ming Lei wrote:
>> 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>
> 
> BTW, there is still some gap(%65 vs. 99%) between this fix and the original
> accounting(before applying Mike/Mikulas's 7 patches), and it might be
> one thing to improve in future.
> 
> 1) test, sda is single queue virtio-scsi, which is emulated by one HDD
> image
> 
> 2) fio test script:
> fio --direct=1 --size=128G --bsrange=4k-4k \
> 		--runtime=20 --numjobs=1 \
> 		--ioengine=libaio --iodepth=16 \
> 		--iodepth_batch_submit=16 \
> 		--iodepth_batch_complete_min=16 \
> 		--group_reporting=1 --filename=/dev/sda \
> 		--name=seq-test --rw=read
> 
> 3) result:
> - v5.6-rc with this patch
> Run status group 0 (all jobs):
>     READ: bw=79.4MiB/s (83.3MB/s), 79.4MiB/s-79.4MiB/s (83.3MB/s-83.3MB/s), io=155
> 88MiB (1665MB), run=20001-20001msec
> 
> Disk stats (read/write):
>    sda: ios=25039/0, merge=375596/0, ticks=18823/0, in_queue=4330, util=99.43%
> 
> 
> - commit 112f158f66cb (which is previous commit of 5b18b5a73760)
> Run status group 0 (all jobs):
>     READ: bw=81.4MiB/s (85.3MB/s), 81.4MiB/s-81.4MiB/s (85.3MB/s-85.3MB/s), io=166
> 28MiB (1707MB), run=20001-20001msec
> 
> Disk stats (read/write):
>    sda: ios=25749/0, merge=386236/0, ticks=17963/0, in_queue=12976, util=65.20%
> 

Yep, my patch guarantees correct time only for queue-depth=1
it accounts jiffies between start -> end (and end -> end).
For queue-depth > 1 jiffies between start -> start are still lost.

Without exact counter for in-flight requests we cannot distinguish
cases start -> start and end -> start

All  we could could is smoothing statistics even more and
account all short gaps as activity:

--- a/block/bio.c
+++ b/block/bio.c
@@ -1769,6 +1769,7 @@ void update_io_ticks(struct hd_struct *part, unsigned long now, bool end)
         stamp = READ_ONCE(part->stamp);
         if (unlikely(stamp != now)) {
                 if (likely(cmpxchg(&part->stamp, stamp, now) == stamp)) {
+                       end |= time_before(now, stamp + HZ / 100);
                         __part_stat_add(part, io_ticks, end ? now - stamp : 1);
                 }
         }


> 
> Thanks,
> Ming
>