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

[Waiman Long]

On 11/2/23 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@xxxxxxxxxx> 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.

Just curious. Does the deadline code remove the deadline quota of an 
exiting task?

Regards,
Longman