[PATCH 2] Avoid preferential treatment of groups that aren'tbacklogged

[Chad Talbott]

Problem: If a group isn't backlogged, we remove it from the service
tree. When it becomes active again, it gets either the minimum vtime
of the tree or gets put at the "back of the line." That is true even
when the group was idle for a very small time, and it consumed some IO
time right before it became idle.  If group has very small weight, it
can end up using more disk time than its fair share. Conversely, if it
has a very large weight, being put at the back of the vtime line is a
large penalty which prevents it consuming its share.

Solution: We solve the problem by assigning the group its old vtime if
it has not been idle long enough. Otherise we assign it the service
tree's min vtime.

Complications: When an entire service tree becomes completely idle, we
lose the vtime state. All the old vtime values are not relevant any
more. For example, consider the case when the service tree is idle and
a brand new group sends IO. That group would have an old vtime value
of zero, but the service tree's vtime would become closer to zero. In
such a case, it would be unfair for the older groups to get a much
higher old vtime stored in them.

We solve that issue by keeping a generation number that counts the
number of instances when the service tree becomes completely
empty. The generation number is stored in each group too. If a group
becomes backlogged after a service tree has been empty, we compare its
stored generation number with the current service tree generation
number, and discard the old vtime if the group generation number is
stale.

The preemption specific code is taken care of automatically because we
allow preemption checks after inserting a group back into the service
tree and assigning it an appropriate vtime.

Signed-off-by: Chad Talbott <ctalbott@xxxxxxxxxx>
---
 block/cfq-iosched.c |   24 +++++++++++++-----------
 1 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 501ffdf..f09f3fe 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -178,6 +178,7 @@ struct cfq_group {
 
 	/* group service_tree key */
 	u64 vdisktime;
+	u64 generation_num;
 	unsigned int weight;
 
 	/* number of cfqq currently on this group */
@@ -300,6 +301,9 @@ struct cfq_data {
 	/* List of cfq groups being managed on this device*/
 	struct hlist_head cfqg_list;
 	struct rcu_head rcu;
+
+	/* Generation number, counts service tree empty events */
+	u64 active_generation;
 };
 
 static struct cfq_group *cfq_get_next_cfqg(struct cfq_data *cfqd);
@@ -873,18 +877,13 @@ cfq_group_service_tree_add(struct cfq_data *cfqd, struct cfq_group *cfqg)
 	if (!RB_EMPTY_NODE(&cfqg->rb_node))
 		return;
 
-	/*
-	 * Currently put the group at the end. Later implement something
-	 * so that groups get lesser vtime based on their weights, so that
-	 * if group does not loose all if it was not continously backlogged.
-	 */
-	n = rb_last(&st->rb);
-	if (n) {
-		__cfqg = rb_entry_cfqg(n);
-		cfqg->vdisktime = __cfqg->vdisktime + CFQ_IDLE_DELAY;
-	} else
+	if (cfqd->active_generation > cfqg->generation_num)
 		cfqg->vdisktime = st->min_vdisktime;
-
+	else
+		/* We assume that vdisktime was not modified when the task
+		   was off the service tree.
+		 */
+		cfqg->vdisktime = max(st->min_vdisktime, cfqg->vdisktime);
 	__cfq_group_service_tree_add(st, cfqg);
 	st->total_weight += cfqg->weight;
 }
@@ -906,6 +905,9 @@ cfq_group_service_tree_del(struct cfq_data *cfqd, struct cfq_group *cfqg)
 	if (!RB_EMPTY_NODE(&cfqg->rb_node))
 		cfq_rb_erase(&cfqg->rb_node, st);
 	cfqg->saved_workload_slice = 0;
+	cfqg->generation_num = cfqd->active_generation;
+	if (RB_EMPTY_ROOT(&st->rb))
+		cfqd->active_generation++;
 	cfq_blkiocg_update_dequeue_stats(&cfqg->blkg, 1);
 }
 

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