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

[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@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.

Cheers,
Longman

Cheers,
Longman