On Tue, Oct 10, 2023 at 10:14 AM T.J. Mercier <tjmercier@xxxxxxxxxx> wrote: > > On Tue, Oct 10, 2023 at 9:31 AM Michal Koutný <mkoutny@xxxxxxxx> wrote: > > > > On Fri, Oct 06, 2023 at 11:37:19AM -0700, "T.J. Mercier" <tjmercier@xxxxxxxxxx> wrote: > > > I suppose it's also possible there is PID reuse by the same app, > > > causing the cgroup to become repopulated at the same time as a kill, > > > but that seems extremely unlikely. Plus, at the point where these > > > kills are occurring we shouldn't normally be simultaneously launching > > > new processes for the app. Similarly if a process forks right before > > > it is killed, maybe it doesn't show up in cgroup.procs until after > > > we've observed it to be empty? > > > > Something like this: > > > > kill (before) > > cgroup_fork > > cgroup_can_fork .. begin(threadgroup_rwsem) > > tasklist_lock > > fatal_signal_pending -> cgroup_cancel_fork kill (mid) > > tasklist_unlock > > seq_start, > > seq_next... > > > > cgroup_post_fork .. end(threadgroup_rwsem) > > kill (after) > > > > Only the third option `kill (after)` means the child would end up on the > > css_set list. But that would mean the reader squeezed before > > cgroup_post_fork() would still see the parent. > > (I.e. I don't see the kill/fork race could skew the listed procs.) > > > So here is a trace from a phone where the kills happen (~100ms) after > the forks. All but one of the children die before we read cgroup.procs > for the first time, and cgroup.procs is not empty. 5ms later we read > again and cgroup.procs is empty, but the last child still hasn't > exited. So it makes sense that the cset from that last child is still > on the list. > https://pastebin.com/raw/tnHhnZBE > Collected a bit more info. It's before exit_mm that the process disappears from cgroup.procs, but the delay to populated=0 seems to be exacerbated by CPU contention during this time. What's weird is that I can see the task blocking the rmdir on cgrp->cset_links->cset->tasks inside of cgroup_destroy_locked when the rmdir is attempted, so I don't understand why it doesn't show up when iterating tasks for cgroup.procs. I'm going to be out until next Wednesday when I'll look some more.
