Re: BUG: HANG_DETECT waiting for migration_cpu_stop() complete

On 9/6/22 16:01, Waiman Long wrote:

On 9/6/22 14:30, Tejun Heo wrote:

quoted

Hello,

(cc'ing Waiman in case he has a better idea)

On Mon, Sep 05, 2022 at 04:22:29PM +0800, Jing-Ting Wu wrote:

quoted

https://lore.kernel.org/lkml/YvrWaml3F+x9Dk+T@slm.duckdns.org/ (local) is for
fix cgroup_threadgroup_rwsem <-> cpus_read_lock() deadlock.
But this issue is cgroup_threadgroup_rwsem <-> cpuset_rwsem deadlock.

If I'm understanding what you're writing correctly, this isn't a 
deadlock.
The cpuset_hotplug_workfn simply isn't being woken up while holding
cpuset_rwsem and others are just waiting for that lock to be released.

I believe it is probably a bug in the scheduler core code. 
__set_cpus_allowed_ptr_locked() calls affine_move_task() to move to a 
random cpu within the new set allowable CPUs. However, if migration is 
disabled, it shouldn't call affine_move_task() at all. Instead, I 
would suggest that if the current cpu is within the new allowable 
cpus, it should just skip doing affine_move_task(). Otherwise, it 
should fail __set_cpus_allowed_ptr_locked().

Maybe like

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 838623b68031..5d9ea1553ec0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c

@@ -2794,9 +2794,9 @@ static int __set_cpus_allowed_ptr_locked(struct 

task_struct *p,
                 if (cpumask_equal(&p->cpus_mask, new_mask))
                         goto out;

-               if (WARN_ON_ONCE(p == current &&
-                                is_migration_disabled(p) &&
-                                !cpumask_test_cpu(task_cpu(p), 
new_mask))) {
+               if (is_migration_disabled(p) &&
+                   !cpumask_test_cpu(task_cpu(p), new_mask)) {
+                       WARN_ON_ONCE(p == current);
                         ret = -EBUSY;
                         goto out;
                 }

@@ -2818,7 +2818,11 @@ static int __set_cpus_allowed_ptr_locked(struct 

task_struct *p,
         if (flags & SCA_USER)
                 user_mask = clear_user_cpus_ptr(p);

-       ret = affine_move_task(rq, p, rf, dest_cpu, flags);
+       if (!is_migration_disabled(p) || (flags & SCA_MIGRATE_ENABLE)) {
+               ret = affine_move_task(rq, p, rf, dest_cpu, flags);
+       } else {
+               task_rq_unlock(rq, p, rf);
+       }

         kfree(user_mask);

I haven't tested it myself, though.

Cheers,
Longman


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