linux-kernel - [PATCH BUGFIX 4/4] block, bfq: invoke flush_idle_tree after reparent_active_queues in pd

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Date:   Sat, 21 Mar 2020 10:45:21 +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,
        bfq-iosched@...glegroups.com, oleksandr@...alenko.name,
        rasibley@...hat.com, vkabatov@...hat.com, xzhou@...hat.com,
        jstancek@...hat.com, Paolo Valente <paolo.valente@...aro.org>,
        cki-project@...hat.com
Subject: [PATCH BUGFIX 4/4] block, bfq: invoke flush_idle_tree after reparent_active_queues in pd_offline

In bfq_pd_offline(), the function bfq_flush_idle_tree() is invoked to
flush the rb tree that contains all idle entities belonging to the pd
(cgroup) being destroyed. In particular, bfq_flush_idle_tree() is
invoked before bfq_reparent_active_queues(). Yet the latter may happen
to add some entities to the idle tree. It happens if, in some of the
calls to bfq_bfqq_move() performed by bfq_reparent_active_queues(),
the queue to move is empty and gets expired.

This commit simply reverses the invocation order between
bfq_flush_idle_tree() and bfq_reparent_active_queues().

Tested-by: cki-project@...hat.com
Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
---
 block/bfq-cgroup.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
index efb89db7ba24..68882b9b8f11 100644
--- a/block/bfq-cgroup.c
+++ b/block/bfq-cgroup.c
@@ -893,13 +893,6 @@ static void bfq_pd_offline(struct blkg_policy_data *pd)
 	for (i = 0; i < BFQ_IOPRIO_CLASSES; i++) {
 		st = bfqg->sched_data.service_tree + i;
 
-		/*
-		 * The idle tree may still contain bfq_queues belonging
-		 * to exited task because they never migrated to a different
-		 * cgroup from the one being destroyed now.
-		 */
-		bfq_flush_idle_tree(st);
-
 		/*
 		 * It may happen that some queues are still active
 		 * (busy) upon group destruction (if the corresponding
@@ -913,6 +906,19 @@ static void bfq_pd_offline(struct blkg_policy_data *pd)
 		 * scheduler has taken no reference.
 		 */
 		bfq_reparent_active_queues(bfqd, bfqg, st, i);
+
+		/*
+		 * The idle tree may still contain bfq_queues
+		 * belonging to exited task because they never
+		 * migrated to a different cgroup from the one being
+		 * destroyed now. In addition, even
+		 * bfq_reparent_active_queues() may happen to add some
+		 * entities to the idle tree. It happens if, in some
+		 * of the calls to bfq_bfqq_move() performed by
+		 * bfq_reparent_active_queues(), the queue to move is
+		 * empty and gets expired.
+		 */
+		bfq_flush_idle_tree(st);
 	}
 
 	__bfq_deactivate_entity(entity, false);
-- 
2.20.1