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From:   "Xiezhipeng (EulerOS)" <xiezhipeng1@huawei.com>
To:     Tejun Heo <tj@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
CC:     Vincent Guittot <vincent.guittot@linaro.org>,
        Sargun Dhillon <sargun@sargun.me>,
        Xiexiuqi <xiexiuqi@huawei.com>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Libin (Huawei)" <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
Thread-Topic: [PATCH] sched: fix infinity loop in update_blocked_averages
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Date:   Fri, 28 Dec 2018 10:25:03 +0000
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Hi Tejun,

On Fri, Dec 28, 2018 10:03 AM, Tejun Heo wrote:
> 
> 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 */
Tested-by: Zhipeng Xie <xiezhipeng1@huawei.com>

This patch fixes things for me, we haven't seen a crash yet.

--
Thanks,
Zhipeng Xie