From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965517AbcAZPWN (ORCPT ); Tue, 26 Jan 2016 10:22:13 -0500 Received: from www.linutronix.de ([62.245.132.108]:37051 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934671AbcAZPWF (ORCPT ); Tue, 26 Jan 2016 10:22:05 -0500 Date: Tue, 26 Jan 2016 16:21:01 +0100 (CET) From: Thomas Gleixner To: Wanpeng Li cc: Peter Zijlstra , Frederic Weisbecker , linux-kernel@vger.kernel.org, Wanpeng Li Subject: Re: [PATCH] nohz: fix jiffies warp while switch to low-res nohz mode In-Reply-To: <1453344859-5235-1-git-send-email-wanpeng.li@hotmail.com> Message-ID: References: <1453344859-5235-1-git-send-email-wanpeng.li@hotmail.com> User-Agent: Alpine 2.11 (DEB 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 21 Jan 2016, Wanpeng Li wrote: > After commit 0ff53d096422 ("tick: sched: Force tick interrupt and get rid > of softirq magic"), next expire time is overwritten by last_jiffies_update > while switch to low-res nohz mode which leads to jiffies warp. This patch > fix it by programming ce device the expire time in the future. > - hrtimer_forward_now(&ts->sched_timer, tick_period); > hrtimer_set_expires(&ts->sched_timer, next); > - tick_program_event(next, 1); > + hrtimer_forward_now(&ts->sched_timer, tick_period); > + tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1); > tick_nohz_activate(ts, NOHZ_MODE_LOWRES); I agree, that the ordering forward/set is wrong and needs to be fixed, but how does that result in a jiffies warp? tick_nohz_handler() tick_sched_do_timer() tick_do_update_jiffies64() delta = ktime_sub(now, last_jiffies_update); if (delta.tv64 < tick_period.tv64) return; So if that timer fires early, tick_do_update_jiffies64() will just return and do nothing. So what makes jiffies warp? Unless I'm missing something, this changelog is patently wrong. Thanks, tglx