Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android

[Sultan Alsawaf]

On Thu, Mar 14, 2019 at 10:47:17AM -0700, Joel Fernandes wrote:
> About the 100ms latency, I wonder whether it is that high because of
> the way Android's lmkd is observing that a process has died. There is
> a gap between when a process memory is freed and when it disappears
> from the process-table.  Once a process is SIGKILLed, it becomes a
> zombie. Its memory is freed instantly during the SIGKILL delivery (I
> traced this so that's how I know), but until it is reaped by its
> parent thread, it will still exist in /proc/<pid> . So if testing the
> existence of /proc/<pid> is how Android is observing that the process
> died, then there can be a large latency where it takes a very long
> time for the parent to actually reap the child way after its memory
> was long freed. A quicker way to know if a process's memory is freed
> before it is reaped could be to read back /proc/<pid>/maps in
> userspace of the victim <pid>, and that file will be empty for zombie
> processes. So then one does not need wait for the parent to reap it. I
> wonder how much of that 100ms you mentioned is actually the "Waiting
> while Parent is reaping the child", than "memory freeing time". So
> yeah for this second problem, the procfds work will help.
>
> By the way another approach that can provide a quick and asynchronous
> notification of when the process memory is freed, is to monitor
> sched_process_exit trace event using eBPF. You can tell eBPF the PID
> that you want to monitor before the SIGKILL. As soon as the process
> dies and its memory is freed, the eBPF program can send a notification
> to user space (using the perf_events polling infra). The
> sched_process_exit fires just after the mmput() happens so it is quite
> close to when the memory is reclaimed. This also doesn't need any
> kernel changes. I could come up with a prototype for this and
> benchmark it on Android, if you want. Just let me know.

Perhaps I'm missing something, but if you want to know when a process has died
after sending a SIGKILL to it, then why not just make the SIGKILL optionally
block until the process has died completely? It'd be rather trivial to just
store a pointer to an onstack completion inside the victim process' task_struct,
and then complete it in free_task().

Thanks,
Sultan