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Message-Id: <20170703080010.1709-2-paolo.valente@linaro.org>
Date: Mon, 3 Jul 2017 10:00:10 +0200
From: Paolo Valente <paolo.valente@...aro.org>
To: Jens Axboe <axboe@...nel.dk>
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
ulf.hansson@...aro.org, broonie@...nel.org,
Paolo Valente <paolo.valente@...aro.org>
Subject: [PATCH BUGFIX] 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@...il.com>
Reported-by: Laurentiu Nicola <lnicola@...d.ro>
Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
Tested-by: Marco Piazza <mpiazza@...il.com>
---
block/bfq-iosched.c | 14 ++++++++++----
block/bfq-iosched.h | 3 ++-
block/bfq-wf2q.c | 39 ++++++++++++++++++++++++++++++++++-----
3 files changed, 46 insertions(+), 10 deletions(-)
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index ed93da2..9d4919b 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -3471,11 +3471,17 @@ 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 5c3bf98..8fd83b8 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 8726ede..5ec05cd 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);
/*
--
2.10.0
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