Re: [PATCH] sched: fix infinity loop in update_blocked_averages

[Sargun Dhillon]

On Thu, Dec 27, 2018 at 9:02 PM Tejun Heo <tj@xxxxxxxxxx> 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@xxxxxxxxx>
We've deployed this patch to our test workload. We haven't seen a crash yet.