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-iosched.c | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-) (limited to 'block/bfq-iosched.c') diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index 12bbc6b8657dd..60a6835265fc3 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); } /* -- cgit v1.2.3