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block, bfq: don't change ioprio class for a bfq_queue on a service tree

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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 <[email protected]>
Reported-by: Laurentiu Nicola <[email protected]>
Signed-off-by: Paolo Valente <[email protected]>
Tested-by: Marco Piazza <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
This commit is contained in:
Paolo Valente 2017-07-03 10:00:10 +02:00 • committed by Jens Axboe
commit 431b17f9d5
3 changed files with 46 additions and 10 deletions
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14
block/bfq-iosched.c
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@ -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);
}
/*