linux-kernel - Re: [PATCH] sched: fix infinity loop in update_blocked

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Message-ID: <CAMp4zn_Nsc58STx+dnzar2+uw=9F3FwSRiK6ZqaBugkcmLk_Xw@mail.gmail.com>
Date:   Fri, 28 Dec 2018 00:38:17 -0500
From:   Sargun Dhillon <sargun@...gun.me>
To:     Tejun Heo <tj@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Xie XiuQi <xiexiuqi@...wei.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, xiezhipeng1@...wei.com,
        huawei.libin@...wei.com,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Dmitry Adamushko <dmitry.adamushko@...il.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH] sched: fix infinity loop in update_blocked_averages

On Thu, Dec 27, 2018 at 9:02 PM Tejun Heo <tj@...nel.org> 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: Sargun Dhillon <sargun@...gun.me>
We've deployed this patch to our test workload. We haven't seen a crash yet.