Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756129AbdC2NEz (ORCPT ); Wed, 29 Mar 2017 09:04:55 -0400 Received: from mail-wr0-f180.google.com ([209.85.128.180]:33884 "EHLO mail-wr0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756020AbdC2NEx (ORCPT ); Wed, 29 Mar 2017 09:04:53 -0400 Date: Wed, 29 Mar 2017 15:04:49 +0200 From: Frederic Weisbecker To: Luiz Capitulino Cc: riel@redhat.com, linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org, Wanpeng Li Subject: Re: [BUG nohz]: wrong user and system time accounting Message-ID: <20170329130447.GB8306@lerouge> References: <20170323165512.60945ac6@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170323165512.60945ac6@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1111 Lines: 35 On Thu, Mar 23, 2017 at 04:55:12PM -0400, Luiz Capitulino wrote: > > When there are two or more tasks executing in user-space and > taking 100% of a nohz_full CPU, top reports 70% system time > and 30% user time utilization. Sometimes I'm even able to get > 100% system time and 0% user time. > > This was reproduced with latest Linus tree (093b995), but I > don't believe it's a regression (at least not a recent one) > as I can reproduce it with older kernels. Also, I have > CONFIG_IRQ_TIME_ACCOUNTING=y and haven't tried to reproduce > without it yet. > > Below you'll find the steps to reproduce and some initial > analysis. > > Steps to reproduce > ------------------ > > 1. Set up a CPU for nohz_full with isolcpus= nohz_full= > > 2. Pin two tasks that hog the CPU 100% of the time to that CPU I failed to reproduce with your config. I'm still getting 99% userspace cputime. So I'm wondering if the hogging style plays a role. I run pure user loops: int main(int argc, char **argv) { for (;;); return 0 } Does your user program perform syscalls or IOs of some sort?