Re: [PATCH v3] sched/fair: Add ancestors of unthrottled undecayed cfs_rq

[Odin Ugedal]

fre. 17. sep. 2021 kl. 16:32 skrev Michal Koutný <mkoutny@xxxxxxxx>:
>
> Since commit a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to
> list on unthrottle") we add cfs_rqs with no runnable tasks but not fully
> decayed into the load (leaf) list. We may ignore adding some ancestors
> and therefore breaking tmp_alone_branch invariant. This broke LTP test
> cfs_bandwidth01 and it was partially fixed in commit fdaba61ef8a2
> ("sched/fair: Ensure that the CFS parent is added after unthrottling").
>
> I noticed the named test still fails even with the fix (but with low
> probability, 1 in ~1000 executions of the test). The reason is when
> bailing out of unthrottle_cfs_rq early, we may miss adding ancestors of
> the unthrottled cfs_rq, thus, not joining tmp_alone_branch properly.
>
> Fix this by adding ancestors if we notice the unthrottled cfs_rq was
> added to the load list.
>
> Fixes: a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
> Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx>

Thanks for catching this!

Reviewed-by: Odin Ugedal <odin@xxxxxxx>

> Changes since v2 [1]:
> - jump to completion for loop, don't duplicate it
> - singled out of the series (to handle a fix separately from the rest)
>
> [1] https://lore.kernel.org/r/20210819175034.4577-2-mkoutny@xxxxxxxx/
>
>  kernel/sched/fair.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ff69f245b939..f6a05d9b5443 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4936,8 +4936,12 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
>         /* update hierarchical throttle state */
>         walk_tg_tree_from(cfs_rq->tg, tg_nop, tg_unthrottle_up, (void *)rq);
>
> -       if (!cfs_rq->load.weight)
> +       /* Nothing to run but something to decay (on_list)? Complete the branch */
> +       if (!cfs_rq->load.weight) {
> +               if (cfs_rq->on_list)
> +                       goto unthrottle_throttle;
>                 return;
> +       }
>
>         task_delta = cfs_rq->h_nr_running;
>         idle_task_delta = cfs_rq->idle_h_nr_running;
> --
> 2.32.0
>