[PATCH 03/25] io-controller: bfq support of in-class preemption

o Generally preemption is associated with cross class where if an request
from RT class is pending it will preempt the ongoing BE or IDLE class
request.
o CFQ also does in-class preemtions like a sync request queue preempting the
async request queue. In that case it looks like preempting queue gains
share and it is not fair.
o Implement the similar functionality in bfq so that we can retain the
existing CFQ behavior.
o This patch creates a bypass path so that a queue can be put at the
front of the service tree (add_front, similar to CFQ), so that it will
be selected next to run. That's a different thing that in the process
this queue gains share.
Signed-off-by: Vivek Goyal
---
block/elevator-fq.c | 43 +++++++++++++++++++++++++++++++++++++++----
1 files changed, 39 insertions(+), 4 deletions(-)
diff --git a/block/elevator-fq.c b/block/elevator-fq.c
index a58efdc..7ee4321 100644
--- a/block/elevator-fq.c
+++ b/block/elevator-fq.c
@@ -582,7 +582,7 @@ static struct io_entity *bfq_lookup_next_entity(struct io_sched_data *sd,
* service received if @entity is active) of the queue to calculate its
* timestamps.
*/
-static void __bfq_activate_entity(struct io_entity *entity)
+static void __bfq_activate_entity(struct io_entity *entity, int add_front)
{
struct io_sched_data *sd = entity->sched_data;
struct io_service_tree *st = io_entity_service_tree(entity);
@@ -627,7 +627,42 @@ static void __bfq_activate_entity(struct io_entity *entity)
}

st = __bfq_entity_update_prio(st, entity);
- bfq_calc_finish(entity, entity->budget);
+ /*
+ * This is to emulate cfq like functionality where preemption can
+ * happen with-in same class, like sync queue preempting async queue
+ * May be this is not a very good idea from fairness point of view
+ * as preempting queue gains share. Keeping it for now.
+ */
+ if (add_front) {
+ struct io_entity *next_entity;
+
+ /*
+ * Determine the entity which will be dispatched next
+ * Use sd->next_active once hierarchical patch is applied
+ */
+ next_entity = bfq_lookup_next_entity(sd, 0);
+
+ if (next_entity && next_entity != entity) {
+ struct io_service_tree *new_st;
+ u64 delta;
+
+ new_st = io_entity_service_tree(next_entity);
+
+ /*
+ * At this point, both entities should belong to
+ * same service tree as cross service tree preemption
+ * is automatically taken care by algorithm
+ */
+ BUG_ON(new_st != st);
+ entity->finish = next_entity->finish - 1;
+ delta = bfq_delta(entity->budget, entity->weight);
+ entity->start = entity->finish - delta;
+ if (bfq_gt(entity->start, st->vtime))
+ entity->start = st->vtime;
+ }
+ } else {
+ bfq_calc_finish(entity, entity->budget);
+ }
bfq_active_insert(st, entity);
}

@@ -635,9 +670,9 @@ static void __bfq_activate_entity(struct io_entity *entity)
* bfq_activate_entity - activate an entity.
* @entity: the entity to activate.
*/
-static void bfq_activate_entity(struct io_entity *entity)
+static void bfq_activate_entity(struct io_entity *entity, int add_front)
{
- __bfq_activate_entity(entity);
+ __bfq_activate_entity(entity, add_front);
}

/**
--
1.6.0.6
--
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