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 <redacted>                                                           
---                                                                                               
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