From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754816AbdC3E1m (ORCPT ); Thu, 30 Mar 2017 00:27:42 -0400 Received: from mout.gmx.net ([212.227.15.15]:49350 "EHLO mout.gmx.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752519AbdC3E1l (ORCPT ); Thu, 30 Mar 2017 00:27:41 -0400 Message-ID: <1490848051.4167.57.camel@gmx.de> Subject: Re: [BUG nohz]: wrong user and system time accounting From: Mike Galbraith To: Rik van Riel , Luiz Capitulino Cc: Wanpeng Li , Frederic Weisbecker , linux-kernel@vger.kernel.org Date: Thu, 30 Mar 2017 06:27:31 +0200 In-Reply-To: <1490818125.28917.11.camel@redhat.com> References: <20170323165512.60945ac6@redhat.com> <1490636129.8850.76.camel@redhat.com> <20170328132406.7d23579c@redhat.com> <20170329131656.1d6cb743@redhat.com> <1490818125.28917.11.camel@redhat.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.16.5 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-Provags-ID: V03:K0:UI3BnHgUwmeqsgFSoFDztHj7ybpGStCrz7Vh3J/bSWu4hJaamZl 0mssFos26wU3K1barkJSQSXYpn/8JkbDiKQNFtzjDVrXE6AbfefRvCZrML/xuSzeSLiGdcd jsJzbB1r1oRMqCnZd00q9C5M/UY3boRVCxatURWTGtQv5w4Yvs5sS9L88Kzxi5wCu6IFGnc 1Ys2OowyB+D1jDYCl6NDg== X-UI-Out-Filterresults: notjunk:1;V01:K0:dKuM7qIo2vY=:L55wCur777Ymk+TVN2X+78 IYRBRYZiv8lZxCIwAtYiUuQetchcR0qeENYWBtga6LVUTirYsNEzSgcdwxk3MycAAcCg5wmr8 mCEEL+8tGQfD+yiT0El+RwAQkenWs982bAP6bYdKho6tvyjdaLlcERshB+fzAiCcmg5GQqn6T c30JaXHBqTrW0bov8paPxu1Ww5jcavtRIcWT7MbC0RE2s5JkYOQwtOsipZcRw4smhrUBK0fJt yn6Q56Dmx+j00EBS4Y8YkkwNBiiNHSJVZ0BWN/i+Fecd8xNMlVLnn2lx4D5QtJlTPs/m6XpkO uT9F7YIu8Y18pmCGTzFG1IqIIzDncVZXRSOSHRFs9D3E+0fUSRlcIjzOWeJ9g/Zgw588n3XJ2 aor4ahFKK3zLVMw/6SDQ0PVW8TaFYEuz8Qgt/5jAerh6sA8Qts89bfP4ewAQ8noeIw8hniAS7 jv5MLAgY6ujZqGVuvkdk9NS71gbq9qlVWxPdzT9Ukci6MjjlysnMc3OVW2QrFxj9YJpDjEGUM W0d633+DqxuBHhzRu1cUu7f/EQTKPXZA78MKbGo8dZhyQsep/MvjGr0Bv+YGeqRp0N+y0NS4M cWXOjtpdjNumPQL0NgTLv1bo0qByMuyBv+kMN/xpqzpcuXjHubzp3ReIkMGnp7Jyx0Q3ekIA6 tVkkMYAPkMw0WVC2UJXApr3nTb3iIkZOLEqGyqNCBKtPt5CYjJpAuunWIQQHO6GT/coK246Qc RYHguEjtWBSPiSWZoqhH3gm4CSCOei5rBhdALG4jbP4JQCCYlsxbuusOGsjETqGh60ZkzsMIK 2TgpHQU Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2017-03-29 at 16:08 -0400, Rik van Riel wrote: > In other words, the tick on cpu0 is aligned > with the tick on the nohz_full cpus, and > jiffies is advanced while the nohz_full cpus > with an active tick happen to be in kernel > mode? You really want skew_tick=1, especially on big boxen. > Frederic, can you think of any reason why > the tick on nohz_full CPUs would end up aligned > with the tick on cpu0, instead of running at some > random offset? (I or low rq->clock bits as crude NOHZ collision avoidance) > A random offset, or better yet a somewhat randomized > tick length to make sure that simultaneous ticks are > fairly rare and the vtime sampling does not end up > "in phase" with the jiffies incrementing, could make > the accounting work right again. That improves jitter, especially on big boxen. I have an 8 socket box that thinks it's an extra large PC, there, collision avoidance matters hugely. I couldn't reproduce bean counting woes, no idea if collision avoidance will help that. -Mike