dm cache: age and write back cache entries even without active IO

The policy tick() method is normally called from interrupt context.
Both the mq and smq policies do some bottom half work for the tick
method in their map functions. However if no IO is going through the
cache, then that bottom half work doesn't occur. With these policies
this means recently hit entries do not age and do not get written
back as early as we'd like.

Fix this by introducing a new 'can_block' parameter to the tick()
method. When this is set the bottom half work occurs immediately.
'can_block' is set when the tick method is called every second by the
core target (not in interrupt context).

Signed-off-by: Joe Thornber <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-cache-policy-internal.h

···
 	return p->residency(p);
 }
 
-static inline void policy_tick(struct dm_cache_policy *p)
+static inline void policy_tick(struct dm_cache_policy *p, bool can_block)
 {
 	if (p->tick)
-		return p->tick(p);
+		return p->tick(p, can_block);
 }
 
 static inline int policy_emit_config_values(struct dm_cache_policy *p, char *result,

drivers/md/dm-cache-policy-mq.c

···
 	return r;
 }
 
-static void mq_tick(struct dm_cache_policy *p)
+static void mq_tick(struct dm_cache_policy *p, bool can_block)
 {
 	struct mq_policy *mq = to_mq_policy(p);
 	unsigned long flags;
···
 	spin_lock_irqsave(&mq->tick_lock, flags);
 	mq->tick_protected++;
 	spin_unlock_irqrestore(&mq->tick_lock, flags);
+
+	if (can_block) {
+		mutex_lock(&mq->lock);
+		copy_tick(mq);
+		mutex_unlock(&mq->lock);
+	}
 }
 
 static int mq_set_config_value(struct dm_cache_policy *p,

drivers/md/dm-cache-policy-smq.c

···
 	return r;
 }
 
-static void smq_tick(struct dm_cache_policy *p)
+static void smq_tick(struct dm_cache_policy *p, bool can_block)
 {
 	struct smq_policy *mq = to_smq_policy(p);
 	unsigned long flags;
···
 	spin_lock_irqsave(&mq->tick_lock, flags);
 	mq->tick_protected++;
 	spin_unlock_irqrestore(&mq->tick_lock, flags);
+
+	if (can_block) {
+		mutex_lock(&mq->lock);
+		copy_tick(mq);
+		mutex_unlock(&mq->lock);
+	}
 }
 
 /* Init the policy plugin interface function pointers. */

drivers/md/dm-cache-policy.h

···
 	 * Because of where we sit in the block layer, we can be asked to
 	 * map a lot of little bios that are all in the same block (no
 	 * queue merging has occurred).  To stop the policy being fooled by
-	 * these the core target sends regular tick() calls to the policy.
+	 * these, the core target sends regular tick() calls to the policy.
 	 * The policy should only count an entry as hit once per tick.
 	 */
-	void (*tick)(struct dm_cache_policy *p);
+	void (*tick)(struct dm_cache_policy *p, bool can_block);
 
 	/*
 	 * Configuration.

drivers/md/dm-cache-target.c

···
 static void do_waker(struct work_struct *ws)
 {
 	struct cache *cache = container_of(to_delayed_work(ws), struct cache, waker);
-	policy_tick(cache->policy);
+	policy_tick(cache->policy, true);
 	wake_worker(cache);
 	queue_delayed_work(cache->wq, &cache->waker, COMMIT_PERIOD);
 }
···
 	struct per_bio_data *pb = get_per_bio_data(bio, pb_data_size);
 
 	if (pb->tick) {
-		policy_tick(cache->policy);
+		policy_tick(cache->policy, false);
 
 		spin_lock_irqsave(&cache->lock, flags);
 		cache->need_tick_bio = true;