Messages in this thread | | | Date | Fri, 3 Nov 2023 13:02:24 -0400 | Subject | Re: [PATCH v2] cgroup/cpuset: Change nr_deadline_tasks to an atomic_t value | From | Waiman Long <> |
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On 11/3/23 10:29, Juri Lelli wrote: > On 02/11/23 09:01, Waiman Long wrote: >> On 11/2/23 06:26, Juri Lelli wrote: >>> 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 for the reminder. I look at your comment again. Even though >> dl_rebuild_rd_accounting() operates on css(es) via css_task_iter_start() and >> css_task_iter_next(), the css_set_lock is released at the end of it. So it >> is still possible that a task can call cgroup_exit() after >> css_task_iter_next() and is being processed by dl_add_task_root_domain(). Is >> there a helper in the do_exit() path to nullify the dl_task() check. Or >> maybe we can also check for PF_EXITING in dl_add_task_root_domain() under >> the pi_lock and do the dl_task() check the under pi_lock to synchronize with >> dl_add_task_root_domain(). What do you think? >> >> I still believe that it doesn't really matter if we call dec_dl_tasks_cs() >> inside or outside the css_set_lock. > Hummm, what if we move dec_dl_tasks_cs outside css_set_lock guard in > cgroup_exit and we grab cpuset_mutex (for dl_tasks) before doing the > decrement in there?
It is a possibility, but acquiring a mutex in the exit path may cause it to sleep. A dying task in the sleep state may be a problem.
Cheers, Longman
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