Re: [RFC] sched/fair: hard lockup in sched_cfs_period_timer

Phil Auld
2019-03-06T16:23:18Z

On Tue, Mar 05, 2019 at 12:45:34PM -0800 bsegall@google.com wrote:
> Phil Auld <pauld@redhat.com> writes:
>
> > Interestingly, if I limit the number of child cgroups to the number of
> > them I'm actually putting processes into (16 down from 2500) the problem
> > does not reproduce.
>
> That is indeed interesting, and definitely not something we'd want to
> matter. (Particularly if it's not root->a->b->c...->throttled_cgroup or
> root->throttled->a->...->thread vs root->throttled_cgroup, which is what
> I was originally thinking of)
>

The locking may be a red herring.

The setup is root->throttled->a where a is 1-2500. There are 4 threads in
each of the first 16 a groups. The parent, throttled, is where the
cfs_period/quota_us are set.

I wonder if the problem is the walk_tg_tree_from() call in unthrottle_cfs_rq().

The distribute_cfg_runtime looks to be O(n * m) where n is number of
throttled cfs_rqs and m is the number of child cgroups. But I'm not
completely clear on how the hierarchical cgroups play together here.

I'll pull on this thread some.

Thanks for your input.

Cheers,
Phil

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