On Fri, Dec 28, 2012 at 12:35:45PM -0800, Tejun Heo wrote:

To support hierarchical stats, it's necessary to remember stats from
dead children.  Add cfqg->dead_stats and make a dying cfqg transfer
its stats to the parent's dead-stats.

Hi Tejun,

Why not directly transfer stats to cfqg->stats. IOW, what's the advantage
of maintaining dead_stats separately.

[..]

+ * Transfer @cfqg's stats to its parent's dead_stats so that the ancestors'
+ * recursive stats can still account for the amount used by this cfqg after
+ * it's gone.
+ */
+static void cfqg_stats_xfer_dead(struct cfq_group *cfqg)
+{
+	struct cfq_group *parent = cfqg_parent(cfqg);
+
+	lockdep_assert_held(cfqg_to_blkg(cfqg)->q->queue_lock);
+
+	if (unlikely(!parent))
+		return;
+
+	cfqg_stats_merge(&parent->dead_stats, &cfqg->stats);
+	cfqg_stats_merge(&parent->dead_stats, &cfqg->dead_stats);
+	cfqg_stats_reset(&cfqg->stats);
+	cfqg_stats_reset(&cfqg->dead_stats);

Anyway group will be marked offline and later freed. So resetting stats
might not be required.

In fact if we have a realiable way of resetting status then online/offline
infrastructure might not be required? I think per cpu stats will be a
problem though and that's why we probably require logic to online/offline
the group?

Thanks
Vivek