Received: by 2002:ac0:950e:0:0:0:0:0 with SMTP id f14csp1119352imc; Sun, 17 Mar 2019 04:43:36 -0700 (PDT) X-Google-Smtp-Source: APXvYqy2LaK70zR7iztFrYEzGFBfdR4s0HW8awNjBUyBeQ4wlRf1Y9/A73IYLfxpX1yPy6nnqDyy X-Received: by 2002:a62:ea10:: with SMTP id t16mr13518760pfh.3.1552823015927; Sun, 17 Mar 2019 04:43:35 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1552823015; cv=none; d=google.com; s=arc-20160816; b=ek2M9afIMXBvXb5hEgi1e6zo+8nUyfOr/J2eVyqSivsAVsXRq4uNl83e6Jguzbd2Wi JqZyjllNGJs7BWk41E6kGXv3/3Q9ysrgUE3SBxT1CwfUalZpD8xR/60PbmOCUUNw0y8a 7AuNjhlEN8NlB17nnwYNcpyA68gdbwLZaRPq5LI1ooVqjlHArGmNimNfG4PenUpx3zVw TFu7plbsoZdthysDiTDzlIRNsVAPWMRHGZ+nQb1OxnEaDPv8Pg0/LwZ52H0w9yy6dKtH VXUO2hwvCpHETWM9V3eA3Kf+tPK3A1++mHNDcE/3bCarUFsKOEMAHbzmMxxySMxpYQW9 5IzA== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:sender:user-agent:in-reply-to :content-disposition:mime-version:references:message-id:subject:cc :to:from:date:dkim-signature; bh=YclABiHuTnjl2QFtYMQH4ry+g51OvyNkv8LG/hhb3tI=; b=qZMwoZ04157OJN4vxorDnazVa9Ctv7yhMDmqBB27trvqdcHCA7uHKY6393O/YeyBXm sARXfbPtE55RiTpkGhoejvsJQ65+V1Lm4pVelFE+I+kPlmjHmOSZ1RyPVRiah+U4tjtD UETUYBMYK1bKacocXzULSd3Dh9n7bdxYItSNEe2EheXIdBoypu4FGUNAAbN1A7WwDeL8 fHpY7AQwW9XrftKVureRzfK67DLKbr19kpslGtOkxXNv76/x0m7d2n3m8glg9YHRiXw6 N2TxpKhacNyn1liTXNDdxjZ2Qj2eXlMnjtnP0OOuTpxQFo+qhEEiGNPnt8CQVcHChwKv LKiQ== ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@brauner.io header.s=google header.b=ZIPxt6SS; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Return-Path: Received: from vger.kernel.org (vger.kernel.org. [209.132.180.67]) by mx.google.com with ESMTP id v16si6546170plo.33.2019.03.17.04.43.20; Sun, 17 Mar 2019 04:43:35 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@brauner.io header.s=google header.b=ZIPxt6SS; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726773AbfCQLmp (ORCPT + 99 others); Sun, 17 Mar 2019 07:42:45 -0400 Received: from mail-ed1-f65.google.com ([209.85.208.65]:37605 "EHLO mail-ed1-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726092AbfCQLmp (ORCPT ); Sun, 17 Mar 2019 07:42:45 -0400 Received: by mail-ed1-f65.google.com with SMTP id m12so11185081edv.4 for ; Sun, 17 Mar 2019 04:42:43 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=brauner.io; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=YclABiHuTnjl2QFtYMQH4ry+g51OvyNkv8LG/hhb3tI=; b=ZIPxt6SSVDiLulatoUNz98X0CNjmV6Ds1efoD235rftaX8w9b8tnnJtqLeyz1SIuvQ ptGQj1RMfH6DURu3CFIqpVPXkPk6p3KuICtI+xBq5XfRL4bpvDmhiKnNpPzrEAdgGvRH 2QAOKkzJ0wpuznKaC2e46KtXSIOAC5FGZWxOI7CiamsYQGiIaLq6bcUbSmSBXXWopv+e +GguqJieiTV5nluiBbBO1OSlwqc1fLYxDOFrOe1sXeThfdrviamX3JAuMoAV7gCE9qV4 t/NW0sxVP8lo5XBfaMxz4+n4nhrOfC29AWyU1xJHGqY3EQ5bkS/EDjHB/6tI63ILC2Er CD2Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=YclABiHuTnjl2QFtYMQH4ry+g51OvyNkv8LG/hhb3tI=; b=rwjEAEL0777U1dSEL4T1cpquVuep/+Ile5/7Y72UySazRodiYF4LWRPNRQ3M+j1Ry2 ZgQtBfg7QSVYusvgcbwcoFud+aLu1gaxdTaxXvSXyRlNlJImifxYjG6M7QzTgU8t1RSk IS0Rrr9OAgjLu+d5Z1pW8SVrfGfbHsoRA0gentX8yEpSbKW/MzjHc99qhec8RIPkAWbo +Dez0UDcvBVIsTbY5nSQ5ep+coU5EqeRsKxu2of6t5dmF+we05iOmecaD6G7PaTd5pIC DLkZ0HTH9Y2H2NfdNn3ICs4zQ7wca9z/81Y+qDiXS2kgqpGK6XlH0cv4Nfwpmc5A/eRt 7SIg== X-Gm-Message-State: APjAAAUehN/WPUxBpwGQtJQbkfw4P9n2OBtU3Ql4iufeZG1iQZJ4qcUl 6iS30eYLjoITJp7yu0JZzlUf263dUjqNvQ== X-Received: by 2002:aa7:db04:: with SMTP id t4mr9116387eds.173.1552822962825; Sun, 17 Mar 2019 04:42:42 -0700 (PDT) Received: from brauner.io ([88.128.80.37]) by smtp.gmail.com with ESMTPSA id n10sm1713459ejl.22.2019.03.17.04.42.41 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Sun, 17 Mar 2019 04:42:42 -0700 (PDT) Date: Sun, 17 Mar 2019 12:42:40 +0100 From: Christian Brauner To: Joel Fernandes Cc: Suren Baghdasaryan , Daniel Colascione , Steven Rostedt , Sultan Alsawaf , Tim Murray , Michal Hocko , Greg Kroah-Hartman , Arve =?utf-8?B?SGrDuG5uZXbDpWc=?= , Todd Kjos , Martijn Coenen , Ingo Molnar , Peter Zijlstra , LKML , "open list:ANDROID DRIVERS" , linux-mm , kernel-team , oleg@redhat.com, luto@amacapital.net, serge@hallyn.com Subject: Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android Message-ID: <20190317114238.ab6tvvovpkpozld5@brauner.io> References: <20190315180306.sq3z645p3hygrmt2@brauner.io> <20190315181324.GA248160@google.com> <20190315182426.sujcqbzhzw4llmsa@brauner.io> <20190315184903.GB248160@google.com> <20190316185726.jc53aqq5ph65ojpk@brauner.io> <20190317015306.GA167393@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190317015306.GA167393@google.com> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Mar 16, 2019 at 09:53:06PM -0400, Joel Fernandes wrote: > On Sat, Mar 16, 2019 at 12:37:18PM -0700, Suren Baghdasaryan wrote: > > On Sat, Mar 16, 2019 at 11:57 AM Christian Brauner wrote: > > > > > > On Sat, Mar 16, 2019 at 11:00:10AM -0700, Daniel Colascione wrote: > > > > On Sat, Mar 16, 2019 at 10:31 AM Suren Baghdasaryan wrote: > > > > > > > > > > On Fri, Mar 15, 2019 at 11:49 AM Joel Fernandes wrote: > > > > > > > > > > > > On Fri, Mar 15, 2019 at 07:24:28PM +0100, Christian Brauner wrote: > > > > > > [..] > > > > > > > > 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? > > > > > > > > > > > > Hmm, going through earlier threads, I believe so now. Here was Daniel's > > > > > > reasoning about avoiding a notification about process death through proc > > > > > > directory fd: http://lkml.iu.edu/hypermail/linux/kernel/1811.0/00232.html > > > > > > > > > > > > May be a dedicated syscall for this would be cleaner after all. > > > > > > > > > > Ah, I wish I've seen that discussion before... > > > > > syscall makes sense and it can be non-blocking and we can use > > > > > select/poll/epoll if we use eventfd. > > > > > > > > Thanks for taking a look. > > > > > > > > > I would strongly advocate for > > > > > non-blocking version or at least to have a non-blocking option. > > > > > > > > Waiting for FD readiness is *already* blocking or non-blocking > > > > according to the caller's desire --- users can pass options they want > > > > to poll(2) or whatever. There's no need for any kind of special > > > > configuration knob or non-blocking option. We already *have* a > > > > non-blocking option that works universally for everything. > > > > > > > > As I mentioned in the linked thread, waiting for process exit should > > > > work just like waiting for bytes to appear on a pipe. Process exit > > > > status is just another blob of bytes that a process might receive. A > > > > process exit handle ought to be just another information source. The > > > > reason the unix process API is so awful is that for whatever reason > > > > the original designers treated processes as some kind of special kind > > > > of resource instead of fitting them into the otherwise general-purpose > > > > unix data-handling API. Let's not repeat that mistake. > > > > > > > > > Something like this: > > > > > > > > > > evfd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC); > > > > > // register eventfd to receive death notification > > > > > pidfd_wait(pid_to_kill, evfd); > > > > > // kill the process > > > > > pidfd_send_signal(pid_to_kill, ...) > > > > > // tend to other things > > > > > > > > Now you've lost me. pidfd_wait should return a *new* FD, not wire up > > > > an eventfd. > > > > > > > > Ok, I probably misunderstood your post linked by Joel. I though your > > original proposal was based on being able to poll a file under > > /proc/pid and then you changed your mind to have a separate syscall > > which I assumed would be a blocking one to wait for process exit. > > Maybe you can describe the new interface you are thinking about in > > terms of userspace usage like I did above? Several lines of code would > > explain more than paragraphs of text. > > Hey, Thanks Suren for the eventfd idea. I agree with Daniel on this. The idea > from Daniel here is to wait for process death and exit events by just > referring to a stable fd, independent of whatever is going on in /proc. > > What is needed is something like this (in highly pseudo-code form): > > pidfd = opendir("/proc/",..); > wait_fd = pidfd_wait(pidfd); > read or poll wait_fd (non-blocking or blocking whichever) > > wait_fd will block until the task has either died or reaped. In both these > cases, it can return a suitable string such as "dead" or "reaped" although an > integer with some predefined meaning is also Ok. > > What that guarantees is, even if the task's PID has been reused, or the task > has already died or already died + reaped, all of these events cannot race > with the code above and the information passed to the user is race-free and > stable / guaranteed. > > An eventfd seems to not fit well, because AFAICS passing the raw PID to > eventfd as in your example would still race since the PID could have been > reused by another process by the time the eventfd is created. > > Also Andy's idea in [1] seems to use poll flags to communicate various tihngs > which is still not as explicit about the PID's status so that's a poor API > choice compared to the explicit syscall. > > I am planning to work on a prototype patch based on Daniel's idea and post something > soon (chatted with Daniel about it and will reference him in the posting as > well), during this posting I will also summarize all the previous discussions > and come up with some tests as well. I hope to have something soon. Having pidfd_wait() return another fd will make the syscall harder to swallow for a lot of people I reckon. What exactly prevents us from making the pidfd itself readable/pollable for the exit staus? They are "special" fds anyway. I would really like to avoid polluting the api with multiple different types of fds if possible. ret = pidfd_wait(pidfd); read or poll pidfd (Note that I'm traveling so my responses might be delayed quite a bit.) (Ccing a few people that might have an opinion here.) Christian