Re: [PATCH v2] cgroup/cpuset: Change nr_deadline_tasks to an atomic_t value

Juri Lelli
2023-11-02T10:26:49Z

Hi Waiman,

On 01/11/23 13:59, Waiman Long wrote:
> On 11/1/23 12:34, Michal Koutný wrote:
> > On Tue, Oct 24, 2023 at 10:18:34AM -0400, Waiman Long <longman@redhat.com> wrote:
> > > The nr_deadline_tasks field in cpuset structure was introduced by
> > > commit 6c24849f5515 ("sched/cpuset: Keep track of SCHED_DEADLINE task
> > > in cpusets"). Unlike nr_migrate_dl_tasks which is only modified under
> > > cpuset_mutex, nr_deadline_tasks can be updated under two different
> > > locks - cpuset_mutex in most cases or css_set_lock in cgroup_exit(). As
> > > a result, data races can happen leading to incorrect nr_deadline_tasks
> > > value.
> > The effect is that dl_update_tasks_root_domain() processes tasks
> > unnecessarily or that it incorrectly skips dl_add_task_root_domain()?
> The effect is that dl_update_tasks_root_domain() may return incorrectly or
> it is doing unnecessary work. Will update the commit log to reflect that.
> >
> > > Since it is not practical to somehow take cpuset_mutex in cgroup_exit(),
> > > the easy way out to avoid this possible race condition is by making
> > > nr_deadline_tasks an atomic_t value.
> > If css_set_lock is useless for this fields and it's going to be atomic,
> > could you please add (presumably) a cleanup that moves dec_dl_tasks_cs()
> > from under css_set_lock in cgroup_exit() to a (new but specific)
> > cpuset_cgrp_subsys.exit() handler?
>
> But css_set_lock is needed for updating other css data. It is true that we
> can move dec_dl_tasks_cs() outside of the lock. I can do that in the next
> version.

Not sure if you had a chance to check my last question/comment on your
previous posting?

https://lore.kernel.org/lkml/ZSjfBWgZf15TchA5@localhost.localdomain/

Thanks,
Juri