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>

1 files changed, 10 insertions, 4 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);
 }
 
 /*