linux-kernel - [PATCH BUGFIX IMPROVEMENT 03/14] block, bfq: make sure queue budgets are not below service received

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Date:   Tue, 29 Jan 2019 12:06:27 +0100
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, linus.walleij@...aro.org,
        broonie@...nel.org, bfq-iosched@...glegroups.com,
        oleksandr@...alenko.name, mancha@...er-research.com,
        Paolo Valente <paolo.valente@...aro.org>
Subject: [PATCH BUGFIX IMPROVEMENT 03/14] block, bfq: make sure queue budgets are not below service received

With some unlucky sequences of events, the function
bfq_updated_next_req updates the current budget of a bfq_queue to a
lower value than the service received by the queue using such a
budget. Unfortunately, if this happens, then the return value of the
function bfq_bfqq_budget_left becomes inconsistent.  This commit
solves this problem by lower-bounding the budget computed in
bfq_updated_next_req to the service currently charged to the queue.

Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
---
 block/bfq-iosched.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 9ea2c4f42501..b0e8006475be 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -907,8 +907,10 @@ static void bfq_updated_next_req(struct bfq_data *bfqd,
 		 */
 		return;
 
-	new_budget = max_t(unsigned long, bfqq->max_budget,
-			   bfq_serv_to_charge(next_rq, bfqq));
+	new_budget = max_t(unsigned long,
+			   max_t(unsigned long, bfqq->max_budget,
+				 bfq_serv_to_charge(next_rq, bfqq)),
+			   entity->service);
 	if (entity->budget != new_budget) {
 		entity->budget = new_budget;
 		bfq_log_bfqq(bfqd, bfqq, "updated next rq: new budget %lu",
-- 
2.20.1