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Subject[PATCH 4.19 12/50] bfq-iosched: Revert "bfq: Fix computation of shallow depth"
Date
From: Lin Feng <linf@wangsu.com>

[ Upstream commit 388c705b95f23f317fa43e6abf9ff07b583b721a ]

This reverts commit 6d4d273588378c65915acaf7b2ee74e9dd9c130a.

bfq.limit_depth passes word_depths[] as shallow_depth down to sbitmap core
sbitmap_get_shallow, which uses just the number to limit the scan depth of
each bitmap word, formula:
scan_percentage_for_each_word = shallow_depth / (1 << sbimap->shift) * 100%

That means the comments's percentiles 50%, 75%, 18%, 37% of bfq are correct.
But after commit patch 'bfq: Fix computation of shallow depth', we use
sbitmap.depth instead, as a example in following case:

sbitmap.depth = 256, map_nr = 4, shift = 6; sbitmap_word.depth = 64.
The resulsts of computed bfqd->word_depths[] are {128, 192, 48, 96}, and
three of the numbers exceed core dirver's 'sbitmap_word.depth=64' limit
nothing.

Signed-off-by: Lin Feng <linf@wangsu.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
block/bfq-iosched.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index b7ad8ac6bb41e..5198ed1b36690 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -5280,13 +5280,13 @@ static unsigned int bfq_update_depths(struct bfq_data *bfqd,
* limit 'something'.
*/
/* no more than 50% of tags for async I/O */
- bfqd->word_depths[0][0] = max(bt->sb.depth >> 1, 1U);
+ bfqd->word_depths[0][0] = max((1U << bt->sb.shift) >> 1, 1U);
/*
* no more than 75% of tags for sync writes (25% extra tags
* w.r.t. async I/O, to prevent async I/O from starving sync
* writes)
*/
- bfqd->word_depths[0][1] = max((bt->sb.depth * 3) >> 2, 1U);
+ bfqd->word_depths[0][1] = max(((1U << bt->sb.shift) * 3) >> 2, 1U);

/*
* In-word depths in case some bfq_queue is being weight-
@@ -5296,9 +5296,9 @@ static unsigned int bfq_update_depths(struct bfq_data *bfqd,
* shortage.
*/
/* no more than ~18% of tags for async I/O */
- bfqd->word_depths[1][0] = max((bt->sb.depth * 3) >> 4, 1U);
+ bfqd->word_depths[1][0] = max(((1U << bt->sb.shift) * 3) >> 4, 1U);
/* no more than ~37% of tags for sync writes (~20% extra tags) */
- bfqd->word_depths[1][1] = max((bt->sb.depth * 6) >> 4, 1U);
+ bfqd->word_depths[1][1] = max(((1U << bt->sb.shift) * 6) >> 4, 1U);

for (i = 0; i < 2; i++)
for (j = 0; j < 2; j++)
--
2.27.0


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