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

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Mar 15, 2019 at 11:49 AM Joel Fernandes <joel@joelfernandes.org> wrote:
>
> On Fri, Mar 15, 2019 at 07:24:28PM +0100, Christian Brauner wrote:
> [..]
> > > why do we want to add a new syscall (pidfd_wait) though? Why not just use
> > > standard poll/epoll interface on the proc fd like Daniel was suggesting.
> > > AFAIK, once the proc file is opened, the struct pid is essentially pinned
> > > even though the proc number may be reused. Then the caller can just poll.
> > > We can add a waitqueue to struct pid, and wake up any waiters on process
> > > death (A quick look shows task_struct can be mapped to its struct pid) and
> > > also possibly optimize it using Steve's TIF flag idea. No new syscall is
> > > needed then, let me know if I missed something?
> >
> > Huh, I thought that Daniel was against the poll/epoll solution?
>
> Hmm, going through earlier threads, I believe so now. Here was Daniel's
> reasoning about avoiding a notification about process death through proc
> directory fd: http://lkml.iu.edu/hypermail/linux/kernel/1811.0/00232.html
>
> May be a dedicated syscall for this would be cleaner after all.

Ah, I wish I've seen that discussion before...
syscall makes sense and it can be non-blocking and we can use
select/poll/epoll if we use eventfd. I would strongly advocate for
non-blocking version or at least to have a non-blocking option.
Something like this:

evfd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
// register eventfd to receive death notification
pidfd_wait(pid_to_kill, evfd);
// kill the process
pidfd_send_signal(pid_to_kill, ...)
// tend to other things
...
// wait for the process to die
poll_wait(evfd, ...);

This simplifies userspace, allows it to wait for multiple events using
epoll and I think kernel implementation will be also quite simple
because it already implements eventfd_signal() that takes care of
waitqueue handling.

If pidfd_send_signal could be extended to have an optional eventfd
parameter then we would not even have to add a new syscall.

> > I have no clear opinion on what is better at the moment since I have
> > been mostly concerned with getting pidfd_send_signal() into shape and
> > was reluctant to put more ideas/work into this if it gets shutdown.
> > Once we have pidfd_send_signal() the wait discussion makes sense.
>
> Ok. It looks like that is almost in though (fingers crossed :)).
>
> thanks,
>
>  - Joel
>