dm cache policy mq: tweak algorithm that decides when to promote a block

Rather than maintaining a separate promote_threshold variable that we
periodically update we now use the hit count of the oldest clean
block.  Also add a fudge factor to discourage demoting dirty blocks.

With some tests this has a sizeable difference, because the old code
was too eager to demote blocks.  For example, device-mapper-test-suite's
git_extract_cache_quick test goes from taking 190 seconds, to 142
(linear on spindle takes 250).

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

2 files changed, 53 insertions, 28 deletions

diff --git a/Documentation/device-mapper/cache-policies.txt b/Documentation/device-mapper/cache-policies.txt
index 66c2774c0c64..7746e5dbfd40 100644
--- a/Documentation/device-mapper/cache-policies.txt
+++ b/Documentation/device-mapper/cache-policies.txt
@@ -58,9 +58,9 @@ since spindles tend to have good bandwidth. The io_tracker counts
 contiguous I/Os to try to spot when the io is in one of these sequential
 modes.
 
-Internally the mq policy maintains a promotion threshold variable.  If
-the hit count of a block not in the cache goes above this threshold it
-gets promoted to the cache.  The read, write and discard promote adjustment
+Internally the mq policy determines a promotion threshold.  If the hit
+count of a block not in the cache goes above this threshold it gets
+promoted to the cache.  The read, write and discard promote adjustment
 tunables allow you to tweak the promotion threshold by adding a small
 value based on the io type.  They default to 4, 8 and 1 respectively.
 If you're trying to quickly warm a new cache device you may wish to
diff --git a/drivers/md/dm-cache-policy-mq.c b/drivers/md/dm-cache-policy-mq.c
index 0e385e40909e..334d098d720d 100644
--- a/drivers/md/dm-cache-policy-mq.c
+++ b/drivers/md/dm-cache-policy-mq.c
@@ -181,24 +181,30 @@ static void queue_shift_down(struct queue *q)
  * Gives us the oldest entry of the lowest popoulated level.  If the first
  * level is emptied then we shift down one level.
  */
-static struct list_head *queue_pop(struct queue *q)
+static struct list_head *queue_peek(struct queue *q)
 {
         unsigned level;
-        struct list_head *r;
 
         for (level = 0; level < NR_QUEUE_LEVELS; level++)
-                if (!list_empty(q->qs + level)) {
-                        r = q->qs[level].next;
-                        list_del(r);
+                if (!list_empty(q->qs + level))
+                        return q->qs[level].next;
 
-                        /* have we just emptied the bottom level? */
-                        if (level == 0 && list_empty(q->qs))
-                                queue_shift_down(q);
+        return NULL;
+}
 
-                        return r;
-                }
+static struct list_head *queue_pop(struct queue *q)
+{
+        struct list_head *r = queue_peek(q);
 
-        return NULL;
+        if (r) {
+                list_del(r);
+
+                /* have we just emptied the bottom level? */
+                if (list_empty(q->qs))
+                        queue_shift_down(q);
+        }
+
+        return r;
 }
 
 static struct list_head *list_pop(struct list_head *lh)
@@ -383,13 +389,6 @@ struct mq_policy {
         unsigned generation;
         unsigned generation_period; /* in lookups (will probably change) */
 
-        /*
-         * Entries in the pre_cache whose hit count passes the promotion
-         * threshold move to the cache proper.  Working out the correct
-         * value for the promotion_threshold is crucial to this policy.
-         */
-        unsigned promote_threshold;
-
         unsigned discard_promote_adjustment;
         unsigned read_promote_adjustment;
         unsigned write_promote_adjustment;
@@ -406,6 +405,7 @@ struct mq_policy {
 #define DEFAULT_DISCARD_PROMOTE_ADJUSTMENT 1
 #define DEFAULT_READ_PROMOTE_ADJUSTMENT 4
 #define DEFAULT_WRITE_PROMOTE_ADJUSTMENT 8
+#define DISCOURAGE_DEMOTING_DIRTY_THRESHOLD 128
 
 /*----------------------------------------------------------------*/
 
@@ -518,6 +518,12 @@ static struct entry *pop(struct mq_policy *mq, struct queue *q)
         return e;
 }
 
+static struct entry *peek(struct queue *q)
+{
+        struct list_head *h = queue_peek(q);
+        return h ? container_of(h, struct entry, list) : NULL;
+}
+
 /*
  * Has this entry already been updated?
  */
@@ -570,10 +576,6 @@ static void check_generation(struct mq_policy *mq)
                                         break;
                         }
                 }
-
-                mq->promote_threshold = nr ? total / nr : 1;
-                if (mq->promote_threshold * nr < total)
-                        mq->promote_threshold++;
         }
 }
 
@@ -641,6 +643,30 @@ static int demote_cblock(struct mq_policy *mq, dm_oblock_t *oblock)
 }
 
 /*
+ * Entries in the pre_cache whose hit count passes the promotion
+ * threshold move to the cache proper.  Working out the correct
+ * value for the promotion_threshold is crucial to this policy.
+ */
+static unsigned promote_threshold(struct mq_policy *mq)
+{
+        struct entry *e;
+
+        if (any_free_cblocks(mq))
+                return 0;
+
+        e = peek(&mq->cache_clean);
+        if (e)
+                return e->hit_count;
+
+        e = peek(&mq->cache_dirty);
+        if (e)
+                return e->hit_count + DISCOURAGE_DEMOTING_DIRTY_THRESHOLD;
+
+        /* This should never happen */
+        return 0;
+}
+
+/*
  * We modify the basic promotion_threshold depending on the specific io.
  *
  * If the origin block has been discarded then there's no cost to copy it
@@ -653,7 +679,7 @@ static unsigned adjusted_promote_threshold(struct mq_policy *mq,
                                            bool discarded_oblock, int data_dir)
 {
         if (data_dir == READ)
-                return mq->promote_threshold + mq->read_promote_adjustment;
+                return promote_threshold(mq) + mq->read_promote_adjustment;
 
         if (discarded_oblock && (any_free_cblocks(mq) || any_clean_cblocks(mq))) {
                 /*
@@ -663,7 +689,7 @@ static unsigned adjusted_promote_threshold(struct mq_policy *mq,
                 return mq->discard_promote_adjustment;
         }
 
-        return mq->promote_threshold + mq->write_promote_adjustment;
+        return promote_threshold(mq) + mq->write_promote_adjustment;
 }
 
 static bool should_promote(struct mq_policy *mq, struct entry *e,
@@ -1230,7 +1256,6 @@ static struct dm_cache_policy *mq_create(dm_cblock_t cache_size,
         mq->tick = 0;
         mq->hit_count = 0;
         mq->generation = 0;
-        mq->promote_threshold = 0;
         mq->discard_promote_adjustment = DEFAULT_DISCARD_PROMOTE_ADJUSTMENT;
         mq->read_promote_adjustment = DEFAULT_READ_PROMOTE_ADJUSTMENT;
         mq->write_promote_adjustment = DEFAULT_WRITE_PROMOTE_ADJUSTMENT;