bcache: Improve bucket_prio() calculation

When deciding what order to reuse buckets we take into account both the bucket's
priority (which indicates lru order) and also the amount of live data in that
bucket. The way they were scaled together wasn't as correct as it could be...
this patch improves and documents it.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>

diff --git a/drivers/md/bcache/alloc.c b/drivers/md/bcache/alloc.c
index bcfd96e2121b4dd18351c6175d0cf5bc31e75c31..c0d37d0824439f2daff4c049ab82dc6b3c6e646e 100644 (file)
--- a/drivers/md/bcache/alloc.c
+++ b/drivers/md/bcache/alloc.c
@@ -168,8 +168,21 @@ static void invalidate_one_bucket(struct cache *ca, struct bucket *b)
        fifo_push(&ca->free_inc, b - ca->buckets);
 }
 
-#define bucket_prio(b)                         \
-       (((unsigned) (b->prio - ca->set->min_prio)) * GC_SECTORS_USED(b))
+/*
+ * Determines what order we're going to reuse buckets, smallest bucket_prio()
+ * first: we also take into account the number of sectors of live data in that
+ * bucket, and in order for that multiply to make sense we have to scale bucket
+ *
+ * Thus, we scale the bucket priorities so that the bucket with the smallest
+ * prio is worth 1/8th of what INITIAL_PRIO is worth.
+ */
+
+#define bucket_prio(b)                                                 \
+({                                                                     \
+       unsigned min_prio = (INITIAL_PRIO - ca->set->min_prio) / 8;     \
+                                                                       \
+       (b->prio - ca->set->min_prio + min_prio) * GC_SECTORS_USED(b);  \
+})
 
 #define bucket_max_cmp(l, r)   (bucket_prio(l) < bucket_prio(r))
 #define bucket_min_cmp(l, r)   (bucket_prio(l) > bucket_prio(r))

diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index 93b84841966524cfef54e3690a1603d4f7567649..0c707e4f4eafc32adcdb3fec5539b2dbe0998a7a 100644 (file)
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -704,7 +704,7 @@ struct bbio {
 };
 
 #define BTREE_PRIO             USHRT_MAX
-#define INITIAL_PRIO           32768
+#define INITIAL_PRIO           32768U
 
 #define btree_bytes(c)         ((c)->btree_pages * PAGE_SIZE)
 #define btree_blocks(b)                                                        \