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Date:   Thu, 27 Dec 2018 18:02:43 -0800
From:   Tejun Heo <tj@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Vincent Guittot <vincent.guittot@linaro.org>,
        Sargun Dhillon <sargun@sargun.me>,
        Xie XiuQi <xiexiuqi@huawei.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>, xiezhipeng1@huawei.com,
        huawei.libin@huawei.com,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Dmitry Adamushko <dmitry.adamushko@gmail.com>,
        Rik van Riel <riel@surriel.com>
Subject: Re: [PATCH] sched: fix infinity loop in update_blocked_averages
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On Thu, Dec 27, 2018 at 05:53:52PM -0800, Tejun Heo wrote:
> Vincent knows that part way better than me but I think the safest way
> would be doing the optimization removal iff tmp_alone_branch is
> already pointing to leaf_cfs_rq_list.  IIUC, it's pointing to
> something else only while a branch is being built and deferring
> optimization removal by an avg update cycle isn't gonna make any
> difference anyway.

So, something like the following.  Xie, can you see whether the
following patch resolves the problem?

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d1907506318a..88b9118b5191 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7698,7 +7698,8 @@ static void update_blocked_averages(int cpu)
 		 * There can be a lot of idle CPU cgroups.  Don't let fully
 		 * decayed cfs_rqs linger on the list.
 		 */
-		if (cfs_rq_is_decayed(cfs_rq))
+		if (cfs_rq_is_decayed(cfs_rq) &&
+		    rq->tmp_alone_branch == &rq->leaf_cfs_rq_list)
 			list_del_leaf_cfs_rq(cfs_rq);
 
 		/* Don't need periodic decay once load/util_avg are null */