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

[Joel Fernandes <joel@joelfernandes.org>]

On Thu, Mar 14, 2019 at 09:36:43PM -0700, Daniel Colascione wrote:
[snip] 
> > If you can solve this with an ebpf program, I
> > strongly suggest you do that instead.
> 
> Regarding process death notification: I will absolutely not support
> putting aBPF and perf trace events on the critical path of core system
> memory management functionality. Tracing and monitoring facilities are
> great for learning about the system, but they were never intended to
> be load-bearing. The proposed eBPF process-monitoring approach is just
> a variant of the netlink proposal we discussed previously on the pidfd
> threads; it has all of its drawbacks. We really need a core system
> call  --- really, we've needed robust process management since the
> creation of unix --- and I'm glad that we're finally getting it.
> Adding new system calls is not expensive; going to great lengths to
> avoid adding one is like calling a helicopter to avoid crossing the
> street. I don't think we should present an abuse of the debugging and
> performance monitoring infrastructure as an alternative to a robust
> and desperately-needed bit of core functionality that's neither hard
> to add nor complex to implement nor expensive to use.

The eBPF-based solution to this would be just as simple while avoiding any
kernel changes. I don't know why you think it is not load-bearing. However, I
agree the proc/pidfd approach is better and can be simpler for some users who
don't want to deal with eBPF - especially since something like this has many
usecases. I was just suggesting the eBPF solution as a better alternative to
the task_struct surgery idea from Sultan since that sounded to me quite
hackish (that could just be my opinion).

> Regarding the proposal for a new kernel-side lmkd: when possible, the
> kernel should provide mechanism, not policy. Putting the low memory
> killer back into the kernel after we've spent significant effort
> making it possible for userspace to do that job. Compared to kernel
> code, more easily understood, more easily debuggable, more easily
> updated, and much safer. If we *can* move something out of the kernel,
> we should. This patch moves us in exactly the wrong direction. Yes, we
> need *something* that sits synchronously astride the page allocation
> path and does *something* to stop a busy beaver allocator that eats
> all the available memory before lmkd, even mlocked and realtime, can
> respond. The OOM killer is adequate for this very rare case.
> 
> With respect to kill timing: Tim is right about the need for two
> levels of policy: first, a high-level process prioritization and
> memory-demand balancing scheme (which is what OOM score adjustment
> code in ActivityManager amounts to); and second, a low-level
> process-killing methodology that maximizes sustainable memory reclaim
> and minimizes unwanted side effects while killing those processes that
> should be dead. Both of these policies belong in userspace --- because
> they *can* be in userspace --- and userspace needs only a few tools,
> most of which already exist, to do a perfectly adequate job.
> 
> We do want killed processes to die promptly. That's why I support
> boosting a process's priority somehow when lmkd is about to kill it.
> The precise way in which we do that --- involving not only actual
> priority, but scheduler knobs, cgroup assignment, core affinity, and
> so on --- is a complex topic best left to userspace. lmkd already has
> all the knobs it needs to implement whatever priority boosting policy
> it wants.
> 
> Hell, once we add a pidfd_wait --- which I plan to work on, assuming
> nobody beats me to it, after pidfd_send_signal lands --- you can
> imagine a general-purpose priority inheritance mechanism expediting
> process death when a high-priority process waits on a pidfd_wait
> handle for a condemned process. You know you're on the right track
> design-wise when you start seeing this kind of elegant constructive
> interference between seemingly-unrelated features. What we don't need
> is some kind of blocking SIGKILL alternative or backdoor event
> delivery system.
> 
> We definitely don't want to have to wait for a process's parent to
> reap it. Instead, we want to wait for it to become a zombie. That's
> why I designed my original exithand patch to fire death notification
> upon transition to the zombie state, not upon process table removal,
> and I expect pidfd_wait (or whatever we call it) to act the same way.

Agreed. Looking forward to the patches. :)

thanks,

 - Joel