bcache: reduce cache_set devices iteration by devices_max_used

Member devices of struct cache_set is used to reference all attached
bcache devices to this cache set. If it is treated as array of pointers,
size of devices[] is indicated by member nr_uuids of struct cache_set.

nr_uuids is calculated in drivers/md/super.c:bch_cache_set_alloc(),
	bucket_bytes(c) / sizeof(struct uuid_entry)
Bucket size is determined by user space tool "make-bcache", by default it
is 1024 sectors (defined in bcache-tools/make-bcache.c:main()). So default
nr_uuids value is 4096 from the above calculation.

Every time when bcache code iterates bcache devices of a cache set, all
the 4096 pointers are checked even only 1 bcache device is attached to the
cache set, that's a wast of time and unncessary.

This patch adds a member devices_max_used to struct cache_set. Its value
is 1 + the maximum used index of devices[] in a cache set. When iterating
all valid bcache devices of a cache set, use c->devices_max_used in
for-loop may reduce a lot of useless checking.

Personally, my motivation of this patch is not for performance, I use it
in bcache debugging, which helps me to narrow down the scape to check
valid bcached devices of a cache set.

Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Michael Lyle <mlyle@lyle.org>
Reviewed-by: Tang Junhui <tang.junhui@zte.com.cn>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 1 insertions, 1 deletions

diff --git a/drivers/md/bcache/writeback.h b/drivers/md/bcache/writeback.h
index 6d26927267f8..f102b1f9bc51 100644
--- a/drivers/md/bcache/writeback.h
+++ b/drivers/md/bcache/writeback.h
@@ -24,7 +24,7 @@ static inline uint64_t  bcache_flash_devs_sectors_dirty(struct cache_set *c)
 
 	mutex_lock(&bch_register_lock);
 
-	for (i = 0; i < c->nr_uuids; i++) {
+	for (i = 0; i < c->devices_max_used; i++) {
 		struct bcache_device *d = c->devices[i];
 
 		if (!d || !UUID_FLASH_ONLY(&c->uuids[i]))