Hi.
On Wed, Nov 06, 2019 at 02:51:30PM -0800, Roman Gushchin guro@fb.com wrote:
Let's fix it by switching from css_tryget_online() to css_tryget().
Is this a safe thing to do? The stack captures a kmem charge path, with css_tryget() it may happen it gets an offlined memcg and carry out charge into it. What happens when e.g. memcg_deactivate_kmem_caches is skipped as a consequence?
The problem is caused by an exiting task which is associated with an offline memcg. We're iterating over and over in the do {} while (!css_tryget_online()) loop, but obviously the memcg won't become online and the exiting task won't be migrated to a live memcg.
As discussed in other replies, the task is not yet exiting. However, the access to memcg isn't through `current` but `mm->owner`, i.e. another task of a threadgroup may have got stuck in an offlined memcg (I don't have a good explanation for that though).
HTH, Michal