Re: [PATCH 1/2] cgroup/cpuset: Make cpuset hotplug processing synchronous

[Waiman Long]

On 4/2/24 10:13, Michal Koutný wrote:
> Hello Waiman.
>
> (I have no opinion on the overall locking reworks, only the bits about
> v1 migrations caught my attention.)
>
> On Mon, Apr 01, 2024 at 10:58:57AM -0400, Waiman Long <longman@xxxxxxxxxx> wrote:
> ...
> > @@ -4383,12 +4377,20 @@ hotplug_update_tasks_legacy(struct cpuset *cs,
> >   	/*
> >   	 * Move tasks to the nearest ancestor with execution resources,
> >   	 * This is full cgroup operation which will also call back into
> > -	 * cpuset. Should be done outside any lock.
> > +	 * cpuset. Execute it asynchronously using workqueue.
>                     ...to avoid deadlock on cpus_read_lock
>
> Is this the reason?
> Also, what happens with the tasks in the window till the migration
> happens?
> Is it handled gracefully that their cpu is gone?

Yes, there is a potential that a cpus_read_lock() may be called leading 
to deadlock. So unless we reverse the current cgroup_mutex --> 
cpu_hotplug_lock ordering, it is not safe to call 
cgroup_transfer_tasks() directly.


> > -	if (is_empty) {
> > -		mutex_unlock(&cpuset_mutex);
> > -		remove_tasks_in_empty_cpuset(cs);
> > -		mutex_lock(&cpuset_mutex);
> > +	if (is_empty && css_tryget_online(&cs->css)) {
> > +		struct cpuset_remove_tasks_struct *s;
> > +
> > +		s = kzalloc(sizeof(*s), GFP_KERNEL);
> Is there a benefit of having a work for each cpuset?
> Instead of traversing whole top_cpuset once in the async work.

We could do that too. It's just that we have the repeat the iteration 
process once the workfn is invoked, but that has the advantage of not 
needing to do memory allocation. I am OK with either way. Let's see what 
other folks think about that.

Cheers,
Longman