[B][SRU][PATCH 4/5] sched/fair: Fix enqueue_task_fair() warning some more

[po-hsu.lin at canonical.com]

From: Phil Auld <pauld at redhat.com>

Buglink: https://bugs.launchpad.net/bugs/1931325

sched/fair: Fix enqueue_task_fair warning some more

The recent patch, fe61468b2cb (sched/fair: Fix enqueue_task_fair warning)
did not fully resolve the issues with the rq->tmp_alone_branch !=
&rq->leaf_cfs_rq_list warning in enqueue_task_fair. There is a case where
the first for_each_sched_entity loop exits due to on_rq, having incompletely
updated the list.  In this case the second for_each_sched_entity loop can
further modify se. The later code to fix up the list management fails to do
what is needed because se does not point to the sched_entity which broke out
of the first loop. The list is not fixed up because the throttled parent was
already added back to the list by a task enqueue in a parallel child hierarchy.

Address this by calling list_add_leaf_cfs_rq if there are throttled parents
while doing the second for_each_sched_entity loop.

Fixes: fe61468b2cb ("sched/fair: Fix enqueue_task_fair warning")
Suggested-by: Vincent Guittot <vincent.guittot at linaro.org>
Signed-off-by: Phil Auld <pauld at redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann at arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot at linaro.org>
Link: https://lkml.kernel.org/r/20200512135222.GC2201@lorien.usersys.redhat.com
(cherry picked from commit b34cb07dde7c2346dec73d053ce926aeaa087303)
Signed-off-by: Po-Hsu Lin <po-hsu.lin at canonical.com>
---
 kernel/sched/fair.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d7acd44..b4d224f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5330,6 +5330,13 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 		/* end evaluation on encountering a throttled cfs_rq */
 		if (cfs_rq_throttled(cfs_rq))
 			goto enqueue_throttle;
+
+               /*
+                * One parent has been throttled and cfs_rq removed from the
+                * list. Add it back to not break the leaf list.
+                */
+               if (throttled_hierarchy(cfs_rq))
+                       list_add_leaf_cfs_rq(cfs_rq);
 	}
 
 enqueue_throttle:
-- 
2.7.4