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

Michal Koutný
2023-11-01T16:35:02Z

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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()?

> 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?

Thanks,
Michal

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