Re: [RFC 1/2] Eliminate over- and under-counting of io_ticks

[Ming Lei]

On Mon, Jun 08, 2020 at 09:07:23PM -0700, Josh Snyder wrote:
> Previously, io_ticks could be under-counted. Consider these I/Os along
> the time axis (in jiffies):
> 
>   t          012345678
>   io1        |----|
>   io2            |---|

In current way, when io2 is done, io tickes should be 5, since 1 tick
is added for two io start.

> 
> Under the old approach, io_ticks would count up to 6, like so:
> 
>   t          012345678
>   io1        |----|
>   io2            |---|
>   stamp      0   45  8
>   io_ticks   1   23  6

Before commit 5b18b5a73760("block: delete part_round_stats and switch to less precise counting"),
io tick is calculated accurately, which is basically:

	(4 - 0) + (5 - 4) + (8 - 5) = 8

> 
> With this change, io_ticks instead counts to 8, eliminating the
> under-counting:
> 
>   t          012345678
>   io1        |----|
>   io2            |---|
>   stamp      0    5  8
>   io_ticks   0    5  8
> 
> It was also possible for io_ticks to be over-counted. Consider a
> workload that issues I/Os deterministically at intervals of 8ms (125Hz).
> If each I/O takes 1ms, then the true utilization is 12.5%. The previous
> implementation will increment io_ticks once for each jiffy in which an
> I/O ends. Since the workload issues an I/O reliably for each jiffy, the
> reported utilization will be 100%. This commit changes the approach such
> that only I/Os which cross a boundary between jiffies are counted. With
> this change, the given workload would count an I/O tick on every eighth
> jiffy, resulting in a (correct) calculated utilization of 12.5%.
> 
> Signed-off-by: Josh Snyder <joshs@xxxxxxxxxxx>
> Fixes: 5b18b5a73760 ("block: delete part_round_stats and switch to less precise counting")
> ---
>  block/blk-core.c | 20 +++++++++++++-------
>  1 file changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index d1b79dfe9540..a0bbd9e099b9 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1396,14 +1396,22 @@ unsigned int blk_rq_err_bytes(const struct request *rq)
>  }
>  EXPORT_SYMBOL_GPL(blk_rq_err_bytes);
>  
> -static void update_io_ticks(struct hd_struct *part, unsigned long now, bool end)
> +static void update_io_ticks(struct hd_struct *part, unsigned long now, unsigned long start)
>  {
>  	unsigned long stamp;
> +	unsigned long elapsed;
>  again:
>  	stamp = READ_ONCE(part->stamp);
>  	if (unlikely(stamp != now)) {
> -		if (likely(cmpxchg(&part->stamp, stamp, now) == stamp))
> -			__part_stat_add(part, io_ticks, end ? now - stamp : 1);
> +		if (likely(cmpxchg(&part->stamp, stamp, now) == stamp)) {
> +			// stamp denotes the last IO to finish
> +			// If this IO started before stamp, then there was overlap between this IO
> +			// and that one. We increment only by the non-overlap time.
> +			// If not, there was no overlap and we increment by our own time,
> +			// disregarding stamp.

Linux kernel's comment style is '/**/'

> +			elapsed = now - (start < stamp ? stamp : start);
> +			__part_stat_add(part, io_ticks, elapsed);

Looks this way of only sampling IO done is smart, io ticks becomes much
more accurate than before.

> +		}
>  	}
>  	if (part->partno) {
>  		part = &part_to_disk(part)->part0;
> @@ -1439,7 +1447,7 @@ void blk_account_io_done(struct request *req, u64 now)
>  		part_stat_lock();
>  		part = req->part;
>  
> -		update_io_ticks(part, jiffies, true);
> +		update_io_ticks(part, jiffies, nsecs_to_jiffies(req->start_time_ns));

jiffies and req->start_time_ns may be from different clock sources, so
I'd suggest to merge the two patches into one.


Thanks, 
Ming