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

[Daniel Colascione]

On Fri, Mar 15, 2019 at 8:56 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Thu, Mar 14, 2019 at 9:37 PM Daniel Colascione <dancol@xxxxxxxxxx> wrote:
> >
> > On Thu, Mar 14, 2019 at 8:16 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> > >
> > > On Thu, 14 Mar 2019 13:49:11 -0700
> > > Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx> wrote:
> > >
> > > > 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().
> > >
> > > How would you implement such a method in userspace? kill() doesn't take
> > > any parameters but the pid of the process you want to send a signal to,
> > > and the signal to send. This would require a new system call, and be
> > > quite a bit of work.
> >
> > That's what the pidfd work is for. Please read the original threads
> > about the motivation and design of that facility.
> >
> > > 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.
> >
> > 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.
>
> When talking about pidfd_wait functionality do you mean something like
> this: https://lore.kernel.org/patchwork/patch/345098/ ? I missed the
> discussion about it, could you please point me to it?

That directory-polling approach came up in the discussion. It's a bad
idea, mostly for API reasons. I'm talking about something more like
https://lore.kernel.org/lkml/20181029175322.189042-1-dancol@xxxxxxxxxx/,
albeit in system call form instead of in the form of a new per-task
proc file.