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Subject[PATCH BUGFIX 4/4] block, bfq: invoke flush_idle_tree after reparent_active_queues in pd_offline
Date
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@redhat.com
Signed-off-by: Paolo Valente <paolo.valente@linaro.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
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