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? > 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. > > In any case, there's a clear path forward here --- general-purpose, > cheap, and elegant --- and we should just focus on doing that instead > of more complex proposals with few advantages. _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
