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

[Daniel Colascione <dancol@google.com>]

On Sat, Mar 16, 2019 at 10:31 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> 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.

Thanks for taking a look.

> I would strongly advocate for
> non-blocking version or at least to have a non-blocking option.

Waiting for FD readiness is *already* blocking or non-blocking
according to the caller's desire --- users can pass options they want
to poll(2) or whatever. There's no need for any kind of special
configuration knob or non-blocking option. We already *have* a
non-blocking option that works universally for everything.

As I mentioned in the linked thread, waiting for process exit should
work just like waiting for bytes to appear on a pipe. Process exit
status is just another blob of bytes that a process might receive. A
process exit handle ought to be just another information source. The
reason the unix process API is so awful is that for whatever reason
the original designers treated processes as some kind of special kind
of resource instead of fitting them into the otherwise general-purpose
unix data-handling API. Let's not repeat that mistake.

> 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

Now you've lost me. pidfd_wait should return a *new* FD, not wire up
an eventfd.

Why? Because the new type FD can report process exit *status*
information (via read(2) after readability signal) as well as this
binary yes-or-no signal *that* a process exited, and this capability
is useful if you want to the pidfd interface to be a good
general-purpose process management facility to replace the awful
wait() family of functions. You can't get an exit status from an
eventfd. Wiring up an eventfd the way you've proposed also complicates
wait-causality information, complicating both tracing and any priority
inheritance we might want in the future (because all the wakeups gets
mixed into the eventfd and you can't unscramble an egg). And for what?
What do we gain by using an eventfd? Is the reason that exit.c would
be able to use eventfd_signal instead of poking a waitqueue directly?
How is that better? With an eventfd, you've increased path length on
process exit *and* complicated the API for no reason.

> ...
> // wait for the process to die
> poll_wait(evfd, ...);
>
> This simplifies userspace

Not relative to an exit handle it doesn't.

>, allows it to wait for multiple events using
> epoll

So does a process exit status handle.

> and I think kernel implementation will be also quite simple
> because it already implements eventfd_signal() that takes care of
> waitqueue handling.

What if there are multiple eventfds registered for the death of a
process? In any case, you need some mechanism to find, upon process
death, a list of waiters, then wake each of them up. That's either a
global search or a search in some list rooted in a task-related
structure (either struct task or one of its friends). Using an eventfd
here adds nothing, since upon death, you need this list search
regardless, and as I mentioned above, eventfd-wiring just makes the
API worse.

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

There is nothing wrong with adding a new system call. I don't know why
there's this idea circulating that adding system calls is something we
should bend over backwards to avoid. It's cheap, and support-wise,
kernel interface is kernel interface. Sending a signal has *nothing*
to do with wiring up some kind of notification and there's no reason
to mingle it with some kind of event registration.