[PATCH v1 03/10] bcache: reduce cache_set devices iteration by devices_max_used

Coly Li
2018-01-03T14:04:00Z

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>
---
drivers/md/bcache/bcache.h | 1 +
drivers/md/bcache/btree.c | 2 +-
drivers/md/bcache/super.c | 9 ++++++---
drivers/md/bcache/writeback.h | 2 +-
4 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index 843877e017e1..83c569942bd0 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -488,6 +488,7 @@ struct cache_set {
int caches_loaded;

struct bcache_device **devices;
+ unsigned devices_max_used;
struct list_head cached_devs;
uint64_t cached_dev_sectors;
struct closure caching;
diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index 81e8dc3dbe5e..bf0d7978bc3d 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -1678,7 +1678,7 @@ static void bch_btree_gc_finish(struct cache_set *c)

/* don't reclaim buckets to which writeback keys point */
rcu_read_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];
struct cached_dev *dc;
struct keybuf_key *w, *n;
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index b4d28928dec5..064efd869017 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -721,6 +721,9 @@ static void bcache_device_attach(struct bcache_device *d, struct cache_set *c,
d->c = c;
c->devices[id] = d;

+ if (id >= c->devices_max_used)
+ c->devices_max_used = id + 1;
+
closure_get(&c->caching);
}

@@ -1261,7 +1264,7 @@ static int flash_devs_run(struct cache_set *c)
struct uuid_entry *u;

for (u = c->uuids;
- u < c->uuids + c->nr_uuids && !ret;
+ u < c->uuids + c->devices_max_used && !ret;
u++)
if (UUID_FLASH_ONLY(u))
ret = flash_dev_run(c, u);
@@ -1427,7 +1430,7 @@ static void __cache_set_unregister(struct closure *cl)

mutex_lock(&bch_register_lock);

- for (i = 0; i < c->nr_uuids; i++)
+ for (i = 0; i < c->devices_max_used; i++)
if (c->devices[i]) {
if (!UUID_FLASH_ONLY(&c->uuids[i]) &&
test_bit(CACHE_SET_UNREGISTERING, &c->flags)) {
@@ -1490,7 +1493,7 @@ struct cache_set *bch_cache_set_alloc(struct cache_sb *sb)
c->bucket_bits = ilog2(sb->bucket_size);
c->block_bits = ilog2(sb->block_size);
c->nr_uuids = bucket_bytes(c) / sizeof(struct uuid_entry);
-
+ c->devices_max_used = 0;
c->btree_pages = bucket_pages(c);
if (c->btree_pages > BTREE_MAX_PAGES)
c->btree_pages = max_t(int, c->btree_pages / 4,
diff --git a/drivers/md/bcache/writeback.h b/drivers/md/bcache/writeback.h
index a9e3ffb4b03c..1d284f3d0363 100644
--- a/drivers/md/bcache/writeback.h
+++ b/drivers/md/bcache/writeback.h
@@ -21,7 +21,7 @@ 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]))
--
2.15.1