On Fri, Mar 15, 2019 at 02:13:24PM -0400, Joel Fernandes wrote: > On Fri, Mar 15, 2019 at 07:03:07PM +0100, Christian Brauner wrote: > > On Thu, Mar 14, 2019 at 09:36:43PM -0700, Daniel Colascione 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 > > > > Daniel, > > > > I've just been talking to Joel. > > I actually "expected" you to work pidfd_wait() after prior > > conversations we had on the pidfd_send_signal() patchsets. :) That's why > > I got a separate git tree on kernel.org since I expect a lot more work > > to come. I hope that Linus still decides to pull pidfd_send_signal() > > before Sunday (For the ones who have missed the link in a prior response > > of mine: > > https://lkml.org/lkml/2019/3/12/439 > > > > This is the first merge window I sent this PR. > > > > The pidfd tree has a branch for-next that is already tracked by Stephen > > in linux-next since the 5.0 merge window. The patches for > > pidfd_send_signal() sit in the pidfd branch. > > I'd be happy to share the tree with you and Joel (We can rename it if > > you prefer I don't care). > > I would really like to centralize this work so that we sort of have a > > "united front" and end up with a coherent api and can send PRs from a > > centralized place: > > https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/ > > I am totally onboard with working together / reviewing this work with you all > on a common tree somewhere (Christian's pidfd tree is fine). I was curious, Excellent. > 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? 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. Thanks! Christian
