Hi Vipin. On Wed, Dec 22, 2021 at 10:53:50PM +0000, Vipin Sharma <vipinsh@xxxxxxxxxx> wrote: > VM worker kthreads can linger in the VM process's cgroup for sometime > after KVM terminates the VM process. Why is it a problem? And how long are we talking about? > A VM process can terminate between the time window of exit_mm() to > cgroup_exit(), leaving only worker kthreads in the cgroup. Even kthreads should eventually have PF_EXITING set, they shouldd be treated as "user-space" zombies by cgroups, i.e. mostly invisible (e.g. it doesn't prevent rmdir'ing the cgroup). (And after the last task_struct reference is gone, the cgroup structs can be released too. Maybe the cause is holding the reference to the KVM worker thread somewhere for too long.) > Moving worker kthreads back to the original cgroup (kthreadd_task's > cgroup) makes sure that cgroup is empty as soon as the main VM process > is terminated. BTW this used to be done for "user-space" tasks too (migrate to root cgroup) but it was replaced with the less transactional "ignore zombies" approach. So this change seems inconsistent. Regards, Michal
