On Tue, Dec 14, 2021, Vipin Sharma wrote: > VM worker kthreads can linger in the VM process's cgroup for sometime > after KVM temrinates the VM process. > > KVM terminates the worker kthreads by calling kthread_stop() which waits > on the signal generated by exit_mm() in do_exit() during kthread's exit. > However, these kthreads are removed from the cgroup using cgroup_exit() > call which happens after exit_mm() in do_exit(). A VM process can > terminate between the time window of exit_mm() to cgroup_exit(), leaving > only worker kthreads in the cgroup. > > 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. > > Signed-off-by: Vipin Sharma <vipinsh@xxxxxxxxxx> > --- > virt/kvm/kvm_main.c | 11 ++++++++++- > 1 file changed, 10 insertions(+), 1 deletion(-) > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index b0f7e6eb00ff..edd304a18f16 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -5785,7 +5785,7 @@ static int kvm_vm_worker_thread(void *context) > init_context = NULL; > > if (err) > - return err; > + goto out; > > /* Wait to be woken up by the spawner before proceeding. */ > kthread_parkme(); > @@ -5793,6 +5793,15 @@ static int kvm_vm_worker_thread(void *context) > if (!kthread_should_stop()) > err = thread_fn(kvm, data); > > +out: > + /* > + * We need to move the kthread back to its original cgroups, so that it > + * doesn't linger in the cgroups of the user process after that has > + * already terminated. exit_mm() in do_exit() signals kthread_stop() to > + * return, whereas, removal of the task from the cgroups happens in > + * cgroup_exit() which happens after exit_mm(). > + */ > + WARN_ON(cgroup_attach_task_all(kthreadd_task, current)); As the build bot noted, kthreadd_task isn't exported, and I doubt you'll convince folks to let you export it. Why is it problematic for the kthread to linger in the cgroup? Conceptually, it's not really wrong. > return err; > } > > > base-commit: d8f6ef45a623d650f9b97e11553adb4978f6aa70 > -- > 2.34.1.173.g76aa8bc2d0-goog >
