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

[Josh Snyder <joshs@netflix.com>]

Previously, io_ticks could be under-counted. Consider these I/Os along
the time axis (in jiffies):

  t          012345678
  io1        |----|
  io2            |---|

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

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@netflix.com>
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.
+			elapsed = now - (start < stamp ? stamp : start);
+			__part_stat_add(part, io_ticks, elapsed);
+		}
 	}
 	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));
 		part_stat_inc(part, ios[sgrp]);
 		part_stat_add(part, nsecs[sgrp], now - req->start_time_ns);
 		part_stat_unlock();
@@ -1456,7 +1464,6 @@ void blk_account_io_start(struct request *rq)
 	rq->part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
 
 	part_stat_lock();
-	update_io_ticks(rq->part, jiffies, false);
 	part_stat_unlock();
 }
 
@@ -1468,7 +1475,6 @@ unsigned long disk_start_io_acct(struct gendisk *disk, unsigned int sectors,
 	unsigned long now = READ_ONCE(jiffies);
 
 	part_stat_lock();
-	update_io_ticks(part, now, false);
 	part_stat_inc(part, ios[sgrp]);
 	part_stat_add(part, sectors[sgrp], sectors);
 	part_stat_local_inc(part, in_flight[op_is_write(op)]);
@@ -1487,7 +1493,7 @@ void disk_end_io_acct(struct gendisk *disk, unsigned int op,
 	unsigned long duration = now - start_time;
 
 	part_stat_lock();
-	update_io_ticks(part, now, true);
+	update_io_ticks(part, now, start_time);
 	part_stat_add(part, nsecs[sgrp], jiffies_to_nsecs(duration));
 	part_stat_local_dec(part, in_flight[op_is_write(op)]);
 	part_stat_unlock();
-- 
2.25.1