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

Hannes Reinecke
2018-01-08T07:12:41Z

On 01/03/2018 03:03 PM, Coly Li wrote:
> 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]))
>
Hmm. This just cries out for using IDA ...
but maybe for a later patchset.

Reviewed-by: Hannes Reinecke <hare@suse.com>

Cheers,

Hannes
--
Dr. Hannes Reinecke Teamlead Storage & Networking
hare@suse.de +49 911 74053 688
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