bcache: set max writeback rate when I/O request is idle

Commit b1092c9af9ed ("bcache: allow quick writeback when backing idle")
allows the writeback rate to be faster if there is no I/O request on a
bcache device. It works well if there is only one bcache device attached
to the cache set. If there are many bcache devices attached to a cache
set, it may introduce performance regression because multiple faster
writeback threads of the idle bcache devices will compete the btree level
locks with the bcache device who have I/O requests coming.

This patch fixes the above issue by only permitting fast writebac when
all bcache devices attached on the cache set are idle. And if one of the
bcache devices has new I/O request coming, minimized all writeback
throughput immediately and let PI controller __update_writeback_rate()
to decide the upcoming writeback rate for each bcache device.

Also when all bcache devices are idle, limited wrieback rate to a small
number is wast of thoughput, especially when backing devices are slower
non-rotation devices (e.g. SATA SSD). This patch sets a max writeback
rate for each backing device if the whole cache set is idle. A faster
writeback rate in idle time means new I/Os may have more available space
for dirty data, and people may observe a better write performance then.

Please note bcache may change its cache mode in run time, and this patch
still works if the cache mode is switched from writeback mode and there
is still dirty data on cache.

Fixes: Commit b1092c9af9ed ("bcache: allow quick writeback when backing idle")
Cc: stable@vger.kernel.org #4.16+
Signed-off-by: Coly Li <colyli@suse.de>
Tested-by: Kai Krakow <kai@kaishome.de>
Tested-by: Stefan Priebe <s.priebe@profihost.ag>
Cc: Michael Lyle <mlyle@lyle.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 3 insertions, 7 deletions

diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index b393b3fd06b6..05f82ff6f016 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -328,13 +328,6 @@ struct cached_dev {
 	 */
 	atomic_t		has_dirty;
 
-	/*
-	 * Set to zero by things that touch the backing volume-- except
-	 * writeback.  Incremented by writeback.  Used to determine when to
-	 * accelerate idle writeback.
-	 */
-	atomic_t		backing_idle;
-
 	struct bch_ratelimit	writeback_rate;
 	struct delayed_work	writeback_rate_update;
 
@@ -515,6 +508,8 @@ struct cache_set {
 	struct cache_accounting accounting;
 
 	unsigned long		flags;
+	atomic_t		idle_counter;
+	atomic_t		at_max_writeback_rate;
 
 	struct cache_sb		sb;
 
@@ -524,6 +519,7 @@ struct cache_set {
 
 	struct bcache_device	**devices;
 	unsigned		devices_max_used;
+	atomic_t		attached_dev_nr;
 	struct list_head	cached_devs;
 	uint64_t		cached_dev_sectors;
 	atomic_long_t		flash_dev_dirty_sectors;