From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755503AbcK2Hns (ORCPT ); Tue, 29 Nov 2016 02:43:48 -0500 Received: from mail-wj0-f193.google.com ([209.85.210.193]:35853 "EHLO mail-wj0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752320AbcK2Hnh (ORCPT ); Tue, 29 Nov 2016 02:43:37 -0500 Subject: Re: RFC: documentation of the autogroup feature [v2] To: Peter Zijlstra References: <7513b0a5-c5d0-3a92-5849-995af22601e4@gmail.com> <1479921075.4306.153.camel@gmx.de> <1480078973.4075.58.camel@gmx.de> <1480089063.4075.80.camel@gmx.de> <20161125161810.GR3092@twins.programming.kicks-ass.net> <2cc53b76-668b-edf1-6aa7-e6cb5a9801e1@gmail.com> <20161125214936.GB3045@worktop.programming.kicks-ass.net> Cc: mtk.manpages@gmail.com, Mike Galbraith , Ingo Molnar , linux-man , lkml , Thomas Gleixner From: "Michael Kerrisk (man-pages)" Message-ID: <755aba1b-9bf4-0277-0628-b27e725ee2f9@gmail.com> Date: Tue, 29 Nov 2016 08:43:33 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 MIME-Version: 1.0 In-Reply-To: <20161125214936.GB3045@worktop.programming.kicks-ass.net> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Peter, On 11/25/2016 10:49 PM, Peter Zijlstra wrote: > On Fri, Nov 25, 2016 at 09:54:05PM +0100, Michael Kerrisk (man-pages) wrote: >> So, part of what I was struggling with was what you meant by cfs-cgroup. >> Do you mean the CFS bandwidth control features added in Linux 3.2? > > Nope, /me digs around for a bit... around here I suppose: > > 68318b8e0b61 ("Hook up group scheduler with control groups") Thanks. The pieces are starting to fall into place now. > 68318b8e0b61 v2.6.24-rc1~151 > > But I really have no idea what that looked like. > > In any case, for the case of autogroup, the behaviour has always been, > autogroups came quite late. This ("the behavior has always been") isn't quite true. Yes, group scheduling has been around since Linux 2.6.24, but in terms of the semantics of the thread nice value, there was no visible change then, *unless* explicit action was taken to create cgroups. The arrival of autogroups in Linux 2.6.38 was different. With this feature enabled (which is the default), task groups were implicitly created *without the user needing to do anything*. Thus, [two terminal windows] == [two task groups] and in those two terminal windows, nice(1) on a CPU-bound command in one terminal did nothing in terms of improving CPU access for a CPU-bound tasks running on the other terminal window. Put more succinctly: in Linux 2.6.38, autogrouping broke nice(1) for many use cases. Once I came to that simple summary it was easy to find multiple reports of problems from users: http://serverfault.com/questions/405092/nice-level-not-working-on-linux http://superuser.com/questions/805599/nice-has-no-effect-in-linux-unless-the-same-shell-is-used https://www.reddit.com/r/linux/comments/1c4jew/nice_has_no_effect/ http://stackoverflow.com/questions/10342470/process-niceness-priority-setting-has-no-effect-on-linux Someone else quickly pointed out to me another such report: https://bbs.archlinux.org/viewtopic.php?id=149553 And when I quickly surveyed a few more or less savvy Linux users in one room, most understood what nice does, but none of them knew about the behavior change wrought by autogroup. I haven't looked at all of the mails in the old threads that discussed the implementation of this feature, but so far none of those that I saw mentioned this behavior change. It's unfortunate that it never even got documented. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/