From 431b17f9d5453533cba7d73e7e40428e4f90b35d Mon Sep 17 00:00:00 2001 From: Paolo Valente Date: Mon, 3 Jul 2017 10:00:10 +0200 Subject: 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 Reported-by: Laurentiu Nicola Signed-off-by: Paolo Valente Tested-by: Marco Piazza Signed-off-by: Jens Axboe --- block/bfq-wf2q.c | 39 ++++++++++++++++++++++++++++++++++----- 1 file changed, 34 insertions(+), 5 deletions(-) (limited to 'block/bfq-wf2q.c') 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); /* -- cgit