[RFC PATCH v2 1/5] sched/fair: Add ancestors of unthrottled undecayed cfs_rq

[Michal Koutný]

Since commit a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to
list on unthrottle") we add cfs_rqs with no runnable tasks but not fully
decayed into the load (leaf) list. We may ignore adding some ancestors
and therefore breaking tmp_alone_branch invariant. This broke LTP test
cfs_bandwidth01 and it was partially fixed in commit fdaba61ef8a2
("sched/fair: Ensure that the CFS parent is added after unthrottling").

I noticed the named test still fails even with the fix (but with low
probability, 1 in ~1000 executions of the test). The reason is when
bailing out of unthrottle_cfs_rq early, we may miss adding ancestors of
the unthrottled cfs_rq, thus, not joining tmp_alone_branch properly.

Fix this by adding ancestors if we notice the unthrottled cfs_rq was
added to the load list.

Fixes: a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx>
---
 kernel/sched/fair.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 44c452072a1b..2c41a9007928 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4898,8 +4898,16 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
 	/* update hierarchical throttle state */
 	walk_tg_tree_from(cfs_rq->tg, tg_nop, tg_unthrottle_up, (void *)rq);
 
-	if (!cfs_rq->load.weight)
+	if (!cfs_rq->load.weight) {
+		/* Nothing to run but something to decay? Complete the branch */
+		if (cfs_rq->on_list)
+			for_each_sched_entity(se) {
+				if (list_add_leaf_cfs_rq(group_cfs_rq(se)))
+					break;
+			}
+		assert_list_leaf_cfs_rq(rq);
 		return;
+	}
 
 	task_delta = cfs_rq->h_nr_running;
 	idle_task_delta = cfs_rq->idle_h_nr_running;
-- 
2.32.0