Messages in this thread | | | Date | Thu, 2 Nov 2023 09:01:38 -0400 | Subject | Re: [PATCH v2] cgroup/cpuset: Change nr_deadline_tasks to an atomic_t value | From | Waiman Long <> |
| |
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.
Cheers, Longman
Cheers, Longman
| |