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authorPaolo Valente <paolo.valente@linaro.org>2017-07-03 10:00:10 +0200
committerJens Axboe <axboe@kernel.dk>2017-07-03 16:50:00 -0600
commit431b17f9d5453533cba7d73e7e40428e4f90b35d (patch)
treee42cf25bcbfbc315f6b4fc3660dce0890cfdbbe2
parent7a69f9c60b49699579f5bfb71f928cceba0afe1a (diff)
block, bfq: don't change ioprio class for a bfq_queue on a service tree
On each deactivation or re-scheduling (after being served) of a bfq_queue, BFQ invokes the function __bfq_entity_update_weight_prio(), to perform pending updates of ioprio, weight and ioprio class for the bfq_queue. BFQ also invokes this function on I/O-request dispatches, to raise or lower weights more quickly when needed, thereby improving latency. However, the entity representing the bfq_queue may be on the active (sub)tree of a service tree when this happens, and, although with a very low probability, the bfq_queue may happen to also have a pending change of its ioprio class. If both conditions hold when __bfq_entity_update_weight_prio() is invoked, then the entity moves to a sort of hybrid state: the new service tree for the entity, as returned by bfq_entity_service_tree(), differs from service tree on which the entity still is. The functions that handle activations and deactivations of entities do not cope with such a hybrid state (and would need to become more complex to cope). This commit addresses this issue by just making __bfq_entity_update_weight_prio() not perform also a possible pending change of ioprio class, when invoked on an I/O-request dispatch for a bfq_queue. Such a change is thus postponed to when __bfq_entity_update_weight_prio() is invoked on deactivation or re-scheduling of the bfq_queue. Reported-by: Marco Piazza <mpiazza@gmail.com> Reported-by: Laurentiu Nicola <lnicola@dend.ro> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Marco Piazza <mpiazza@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
-rw-r--r--block/bfq-iosched.c14
-rw-r--r--block/bfq-iosched.h3
-rw-r--r--block/bfq-wf2q.c39
3 files changed, 46 insertions, 10 deletions
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 12bbc6b8657d..60a6835265fc 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -3483,11 +3483,17 @@ static void bfq_update_wr_data(struct bfq_data *bfqd, struct bfq_queue *bfqq)
}
}
}
- /* Update weight both if it must be raised and if it must be lowered */
+ /*
+ * To improve latency (for this or other queues), immediately
+ * update weight both if it must be raised and if it must be
+ * lowered. Since, entity may be on some active tree here, and
+ * might have a pending change of its ioprio class, invoke
+ * next function with the last parameter unset (see the
+ * comments on the function).
+ */
if ((entity->weight > entity->orig_weight) != (bfqq->wr_coeff > 1))
- __bfq_entity_update_weight_prio(
- bfq_entity_service_tree(entity),
- entity);
+ __bfq_entity_update_weight_prio(bfq_entity_service_tree(entity),
+ entity, false);
}
/*
diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
index 5c3bf9861492..8fd83b885774 100644
--- a/block/bfq-iosched.h
+++ b/block/bfq-iosched.h
@@ -892,7 +892,8 @@ void bfq_put_idle_entity(struct bfq_service_tree *st,
struct bfq_entity *entity);
struct bfq_service_tree *
__bfq_entity_update_weight_prio(struct bfq_service_tree *old_st,
- struct bfq_entity *entity);
+ struct bfq_entity *entity,
+ bool update_class_too);
void bfq_bfqq_served(struct bfq_queue *bfqq, int served);
void bfq_bfqq_charge_time(struct bfq_data *bfqd, struct bfq_queue *bfqq,
unsigned long time_ms);
diff --git a/block/bfq-wf2q.c b/block/bfq-wf2q.c
index 8726ede19eef..5ec05cd42b80 100644
--- a/block/bfq-wf2q.c
+++ b/block/bfq-wf2q.c
@@ -694,10 +694,28 @@ struct bfq_service_tree *bfq_entity_service_tree(struct bfq_entity *entity)
return sched_data->service_tree + idx;
}
-
+/*
+ * Update weight and priority of entity. If update_class_too is true,
+ * then update the ioprio_class of entity too.
+ *
+ * The reason why the update of ioprio_class is controlled through the
+ * last parameter is as follows. Changing the ioprio class of an
+ * entity implies changing the destination service trees for that
+ * entity. If such a change occurred when the entity is already on one
+ * of the service trees for its previous class, then the state of the
+ * entity would become more complex: none of the new possible service
+ * trees for the entity, according to bfq_entity_service_tree(), would
+ * match any of the possible service trees on which the entity
+ * is. Complex operations involving these trees, such as entity
+ * activations and deactivations, should take into account this
+ * additional complexity. To avoid this issue, this function is
+ * invoked with update_class_too unset in the points in the code where
+ * entity may happen to be on some tree.
+ */
struct bfq_service_tree *
__bfq_entity_update_weight_prio(struct bfq_service_tree *old_st,
- struct bfq_entity *entity)
+ struct bfq_entity *entity,
+ bool update_class_too)
{
struct bfq_service_tree *new_st = old_st;
@@ -739,9 +757,15 @@ __bfq_entity_update_weight_prio(struct bfq_service_tree *old_st,
bfq_weight_to_ioprio(entity->orig_weight);
}
- if (bfqq)
+ if (bfqq && update_class_too)
bfqq->ioprio_class = bfqq->new_ioprio_class;
- entity->prio_changed = 0;
+
+ /*
+ * Reset prio_changed only if the ioprio_class change
+ * is not pending any longer.
+ */
+ if (!bfqq || bfqq->ioprio_class == bfqq->new_ioprio_class)
+ entity->prio_changed = 0;
/*
* NOTE: here we may be changing the weight too early,
@@ -867,7 +891,12 @@ static void bfq_update_fin_time_enqueue(struct bfq_entity *entity,
{
struct bfq_queue *bfqq = bfq_entity_to_bfqq(entity);
- st = __bfq_entity_update_weight_prio(st, entity);
+ /*
+ * When this function is invoked, entity is not in any service
+ * tree, then it is safe to invoke next function with the last
+ * parameter set (see the comments on the function).
+ */
+ st = __bfq_entity_update_weight_prio(st, entity, true);
bfq_calc_finish(entity, entity->budget);
/*