Re: [PATCH] block/blk-stat: fix mean loss when re-summing aggregated stats

[Tang Yizhou]

On 20/7/26 9:38 pm, Tao Cui wrote:
> From: Tao Cui <cuitao@xxxxxxxxxx>
> 
> blk_rq_stat_sum() folds src into dst but only advances dst->mean and
> dst->nr_samples, leaving dst->batch untouched.  The mean is computed
> from src->batch (the raw per-cpu sum), so a stat that has already been
> through one sum carries batch=0; if that aggregated stat is then used
> as the src of another sum, its samples add nothing to the new mean.
> 
> iolatency hits exactly that: iolatency_check_latencies() first sums the
> per-cpu stats into a local stat, then sums that local stat into
> iolat->cur_stat.  After the first sum the local stat has batch=0, so
> every later window drives cur_stat->mean toward zero.  On non-SSD

Good catch, this is indeed a real issue.

> devices it stays at 0, making the latency_sum_ok(&cur_stat) check that
> gates scaling up always true -- the scale-up hysteresis is effectively
> defeated.  SSD devices use the percentile path and are unaffected.
> 
> Keep dst->batch in sync across sums so an aggregated stat can be reused
> as a src.  blk_rq_stat.batch is internal to blk_rq_stat_init/_add/_sum
> (no other reader in the tree), so the wbt and blk-mq consumers, which
> only read ->mean/->min/->nr_samples, behave as before.
> 
> Fixes: 34dbad5d26e2 ("blk-stat: convert to callback-based statistics reporting")
> Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
> 
> ---
>  block/blk-stat.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/block/blk-stat.c b/block/blk-stat.c
> index de126e1ea5ac..4d4781350083 100644
> --- a/block/blk-stat.c
> +++ b/block/blk-stat.c
> @@ -36,6 +36,7 @@ void blk_rq_stat_sum(struct blk_rq_stat *dst, struct blk_rq_stat *src)
>  	dst->mean = div_u64(src->batch + dst->mean * dst->nr_samples,
>  				dst->nr_samples + src->nr_samples);
>  
> +	dst->batch += src->batch;

This change doesn't actually fix the issue. As you already found, the local
stat's batch is 0. Also, according to the comment of blk_rq_stat_sum(), @src is
a per-CPU stat, so there is no issue in blk_stat_timer_fn(). Your change would
actually make this case look strange.

-- 
Best Regards,
Yi

>  	dst->nr_samples += src->nr_samples;
>  }
>  
> --
> 2.43.0
>