From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.6 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_PASS,URIBL_BLOCKED,USER_IN_DEF_DKIM_WL autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E2D23C10F00 for ; Fri, 15 Mar 2019 15:56:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A8660218C3 for ; Fri, 15 Mar 2019 15:56:55 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="sQKSe3w+" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729425AbfCOP4y (ORCPT ); Fri, 15 Mar 2019 11:56:54 -0400 Received: from mail-it1-f193.google.com ([209.85.166.193]:37236 "EHLO mail-it1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727826AbfCOP4x (ORCPT ); Fri, 15 Mar 2019 11:56:53 -0400 Received: by mail-it1-f193.google.com with SMTP id z124so11614912itc.2 for ; Fri, 15 Mar 2019 08:56:52 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=H4rtgT9B6M1B9LQmL3+oXFzPJ5+Ql+wZXXQfq4r7qxM=; b=sQKSe3w+XoNEOHxv44wL0wTqwONaxtUvQC34Q2Nez1GqGkGvEXukH/JAod5bOJXGHm OKGDpaVbIA/dn/74y/fofwr/x6nLERCAStBftMQvXXcbmjhz98E7nUE+wwdzHn3khfx+ 9FC1ns2uL6HfNp+q032A5iCx39NEwhkRkYXDmt5pB6fDuk8LyAj1frF2LnOOv6lj8lgD 592ejppi3Gh6xhesU26P0CiVMsDdMDAR076ZgzCHEKe4oXwnXT7UB2gksgsfbwbQpcPB An5ekgh1mI2jscsUv9feC58UwwOfUQdGBYKOUdtUjOyki0GE6ap6da45meACeTZmwcCV sPRg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=H4rtgT9B6M1B9LQmL3+oXFzPJ5+Ql+wZXXQfq4r7qxM=; b=nCAnMsZyeAhomRDc+eubklCDbmu3W8B56fG0CXpHHlFR2YlZUaqltvYdvLKYnucYRK ljDBnAv+CqSVg4zfGtC+F5IAFVgpylt3gCA/KrgYo4HB73hivwSdLldo9HRkbGn6v26W a/yL2EptyaG/PTcQigvpZx8yesXKhlTtglrZrbXFhrC2iwtgXUwFJ5HLe6qzdikR+F1p 3R8pERaZNUl+GDXqjvr++kEwODSz97m4zWHzbnXJi17sRbyMrSs6NaYX5h/RCumZyf9D A/s6fBUFBZXF+R8yhB3k3Fz3VpbfIXqqQ2AA1pnI2I036JzyZn+y5C5JfOksnFy1GHR4 taqw== X-Gm-Message-State: APjAAAUTZFSJoJ4TBv0m2sNr7fNxLtSKIwEQKXmGQhuJXK2UjpxqVxG5 9p8kqdLa5ZmfsSUjQ9hosm2rYMf3UpRAy9fH0el9Dw== X-Google-Smtp-Source: APXvYqyqcIlbt1RfAFrK2X9pXdPK7c9C3TBI78IspfigtRt/4QFN//wL64B3tnVby9qAAl/BFark1nkjyptSDxFdTH8= X-Received: by 2002:a24:a81:: with SMTP id 123mr2553188itw.43.1552665412172; Fri, 15 Mar 2019 08:56:52 -0700 (PDT) MIME-Version: 1.0 References: <20190310203403.27915-1-sultan@kerneltoast.com> <20190311174320.GC5721@dhcp22.suse.cz> <20190311175800.GA5522@sultan-box.localdomain> <20190311204626.GA3119@sultan-box.localdomain> <20190312080532.GE5721@dhcp22.suse.cz> <20190312163741.GA2762@sultan-box.localdomain> <20190314204911.GA875@sultan-box.localdomain> <20190314231641.5a37932b@oasis.local.home> In-Reply-To: From: Suren Baghdasaryan Date: Fri, 15 Mar 2019 08:56:40 -0700 Message-ID: Subject: Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android To: Daniel Colascione Cc: Steven Rostedt , Sultan Alsawaf , Joel Fernandes , Tim Murray , Michal Hocko , Greg Kroah-Hartman , =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= , Todd Kjos , Martijn Coenen , Christian Brauner , Ingo Molnar , Peter Zijlstra , LKML , "open list:ANDROID DRIVERS" , linux-mm , kernel-team Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 14, 2019 at 9:37 PM Daniel Colascione wrote: > > On Thu, Mar 14, 2019 at 8:16 PM Steven Rostedt wrote: > > > > On Thu, 14 Mar 2019 13:49:11 -0700 > > Sultan Alsawaf 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.