linux-kernel - [PATCH] dm writecache: fix inaccurate reads/writes stats

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Message-Id: <20220706093146.1961598-1-yukuai1@huaweicloud.com>
Date:   Wed,  6 Jul 2022 17:31:46 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     agk@...hat.com, snitzer@...nel.org, dm-devel@...hat.com,
        mpatocka@...hat.com
Cc:     linux-kernel@...r.kernel.org, yukuai3@...wei.com,
        yukuai1@...weicloud.com, yi.zhang@...wei.com
Subject: [PATCH] dm writecache: fix inaccurate reads/writes stats

From: Yu Kuai <yukuai3@...wei.com>

Test procedures:

1) format a dm writecache device with 4k blocksize.
2) flush cache.
3) cache 1G data through write.
4) clear stats.
5) read 2G data with bs=1m.
6) read stats.

Expected result:
cache hit ratio is 50%.

Test result:
stats: 0, 1011345, 749201, 0, 263168, 262144, 0, 0, 0, 0, 0, 0, 0, 0
ratio is 99% (262144/263168)

The way that reads is accounted is different between cache hit and cache
miss:

1) If cache hit, reads will be accounted for each entry, which means reads
and read_hits will both increase 256 for each io in the above test.

2) If cache miss, reads will only account once, which means reads will only
increase 1 for each io in the above test.

The case that writes_around has the same problem, fix it by adding
appropriate reads/writes in writecache_map_remap_origin().

Fixes: e3a35d03407c ("dm writecache: add event counters")
Signed-off-by: Yu Kuai <yukuai3@...wei.com>
---
 drivers/md/dm-writecache.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/drivers/md/dm-writecache.c b/drivers/md/dm-writecache.c
index d74c5a7a0ab4..c2c6c3a023dd 100644
--- a/drivers/md/dm-writecache.c
+++ b/drivers/md/dm-writecache.c
@@ -1329,16 +1329,29 @@ enum wc_map_op {
 	WC_MAP_ERROR,
 };
 
-static enum wc_map_op writecache_map_remap_origin(struct dm_writecache *wc, struct bio *bio,
-						  struct wc_entry *e)
+static enum wc_map_op writecache_map_remap_origin(struct dm_writecache *wc,
+						  struct bio *bio,
+						  struct wc_entry *e, bool read)
 {
+	sector_t next_boundary;
+	unsigned long long miss_count;
+
 	if (e) {
-		sector_t next_boundary =
+		next_boundary =
 			read_original_sector(wc, e) - bio->bi_iter.bi_sector;
 		if (next_boundary < bio->bi_iter.bi_size >> SECTOR_SHIFT)
 			dm_accept_partial_bio(bio, next_boundary);
+	} else {
+		next_boundary = bio->bi_iter.bi_size;
 	}
 
+	miss_count = (round_up(next_boundary, wc->block_size) >>
+			wc->block_size_bits) - 1;
+	if (read)
+		wc->stats.reads += miss_count;
+	else
+		wc->stats.writes += miss_count;
+
 	return WC_MAP_REMAP_ORIGIN;
 }
 
@@ -1366,7 +1379,7 @@ static enum wc_map_op writecache_map_read(struct dm_writecache *wc, struct bio *
 			map_op = WC_MAP_REMAP;
 		}
 	} else {
-		map_op = writecache_map_remap_origin(wc, bio, e);
+		map_op = writecache_map_remap_origin(wc, bio, e, true);
 	}
 
 	return map_op;
@@ -1458,7 +1471,8 @@ static enum wc_map_op writecache_map_write(struct dm_writecache *wc, struct bio
 direct_write:
 				wc->stats.writes_around++;
 				e = writecache_find_entry(wc, bio->bi_iter.bi_sector, WFE_RETURN_FOLLOWING);
-				return writecache_map_remap_origin(wc, bio, e);
+				return writecache_map_remap_origin(wc, bio, e,
+								   false);
 			}
 			wc->stats.writes_blocked_on_freelist++;
 			writecache_wait_on_freelist(wc);
-- 
2.31.1