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
