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>

1 files changed, 15 insertions, 2 deletions

diff --git a/drivers/md/bcache/alloc.c b/drivers/md/bcache/alloc.c
index bcfd96e2121b..c0d37d082443 100644
--- 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))