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

[Tao Cui]

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