[PATCH 5.19 667/717] bcache: fix set_at_max_writeback_rate() for multiple attached devices

Greg Kroah-Hartman
2022-10-22T09:03:07Z

From: Coly Li <colyli@suse.de>

[ Upstream commit d2d05b88035d2d51a5bb6c5afec88a0880c73df4 ]

Inside set_at_max_writeback_rate() the calculation in following if()
check is wrong,
if (atomic_inc_return(&c->idle_counter) <
atomic_read(&c->attached_dev_nr) * 6)

Because each attached backing device has its own writeback thread
running and increasing c->idle_counter, the counter increates much
faster than expected. The correct calculation should be,
(counter / dev_nr) < dev_nr * 6
which equals to,
counter < dev_nr * dev_nr * 6

This patch fixes the above mistake with correct calculation, and helper
routine idle_counter_exceeded() is added to make code be more clear.

Reported-by: Mingzhe Zou <mingzhe.zou@easystack.cn>
Signed-off-by: Coly Li <colyli@suse.de>
Acked-by: Mingzhe Zou <mingzhe.zou@easystack.cn>
Link: https://lore.kernel.org/r/20220919161647.81238-6-colyli@suse.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/md/bcache/writeback.c | 73 +++++++++++++++++++++++++----------
1 file changed, 52 insertions(+), 21 deletions(-)

diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index 3f0ff3aab6f2..9c227e4a8465 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -157,6 +157,53 @@ static void __update_writeback_rate(struct cached_dev *dc)
dc->writeback_rate_target = target;
}

+static bool idle_counter_exceeded(struct cache_set *c)
+{
+ int counter, dev_nr;
+
+ /*
+ * If c->idle_counter is overflow (idel for really long time),
+ * reset as 0 and not set maximum rate this time for code
+ * simplicity.
+ */
+ counter = atomic_inc_return(&c->idle_counter);
+ if (counter <= 0) {
+ atomic_set(&c->idle_counter, 0);
+ return false;
+ }
+
+ dev_nr = atomic_read(&c->attached_dev_nr);
+ if (dev_nr == 0)
+ return false;
+
+ /*
+ * c->idle_counter is increased by writeback thread of all
+ * attached backing devices, in order to represent a rough
+ * time period, counter should be divided by dev_nr.
+ * Otherwise the idle time cannot be larger with more backing
+ * device attached.
+ * The following calculation equals to checking
+ * (counter / dev_nr) < (dev_nr * 6)
+ */
+ if (counter < (dev_nr * dev_nr * 6))
+ return false;
+
+ return true;
+}
+
+/*
+ * Idle_counter is increased every time when update_writeback_rate() is
+ * called. If all backing devices attached to the same cache set have
+ * identical dc->writeback_rate_update_seconds values, it is about 6
+ * rounds of update_writeback_rate() on each backing device before
+ * c->at_max_writeback_rate is set to 1, and then max wrteback rate set
+ * to each dc->writeback_rate.rate.
+ * In order to avoid extra locking cost for counting exact dirty cached
+ * devices number, c->attached_dev_nr is used to calculate the idle
+ * throushold. It might be bigger if not all cached device are in write-
+ * back mode, but it still works well with limited extra rounds of
+ * update_writeback_rate().
+ */
static bool set_at_max_writeback_rate(struct cache_set *c,
struct cached_dev *dc)
{
@@ -167,21 +214,8 @@ static bool set_at_max_writeback_rate(struct cache_set *c,
/* Don't set max writeback rate if gc is running */
if (!c->gc_mark_valid)
return false;
- /*
- * Idle_counter is increased everytime when update_writeback_rate() is
- * called. If all backing devices attached to the same cache set have
- * identical dc->writeback_rate_update_seconds values, it is about 6
- * rounds of update_writeback_rate() on each backing device before
- * c->at_max_writeback_rate is set to 1, and then max wrteback rate set
- * to each dc->writeback_rate.rate.
- * In order to avoid extra locking cost for counting exact dirty cached
- * devices number, c->attached_dev_nr is used to calculate the idle
- * throushold. It might be bigger if not all cached device are in write-
- * back mode, but it still works well with limited extra rounds of
- * update_writeback_rate().
- */
- if (atomic_inc_return(&c->idle_counter) <
- atomic_read(&c->attached_dev_nr) * 6)
+
+ if (!idle_counter_exceeded(c))
return false;

if (atomic_read(&c->at_max_writeback_rate) != 1)
@@ -195,13 +229,10 @@ static bool set_at_max_writeback_rate(struct cache_set *c,
dc->writeback_rate_change = 0;

/*
- * Check c->idle_counter and c->at_max_writeback_rate agagain in case
- * new I/O arrives during before set_at_max_writeback_rate() returns.
- * Then the writeback rate is set to 1, and its new value should be
- * decided via __update_writeback_rate().
+ * In case new I/O arrives during before
+ * set_at_max_writeback_rate() returns.
*/
- if ((atomic_read(&c->idle_counter) <
- atomic_read(&c->attached_dev_nr) * 6) ||
+ if (!idle_counter_exceeded(c) ||
!atomic_read(&c->at_max_writeback_rate))
return false;

--
2.35.1