From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753630AbdAELuD (ORCPT ); Thu, 5 Jan 2017 06:50:03 -0500 Received: from merlin.infradead.org ([205.233.59.134]:46812 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762257AbdAELts (ORCPT ); Thu, 5 Jan 2017 06:49:48 -0500 Date: Thu, 5 Jan 2017 12:49:35 +0100 From: Peter Zijlstra To: Frederic Weisbecker Cc: Thomas Gleixner , LKML , James Hartsock , Wanpeng Li , Ingo Molnar , Rik van Riel Subject: Re: [PATCH v2] nohz: Fix collision between tick and other hrtimers Message-ID: <20170105114935.GK3093@worktop> References: <1483539124-5693-1-git-send-email-fweisbec@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1483539124-5693-1-git-send-email-fweisbec@gmail.com> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 04, 2017 at 03:12:04PM +0100, Frederic Weisbecker wrote: > When the tick is stopped and an interrupt occurs afterward, we check on > that interrupt exit if the next tick needs to be rescheduled. If it > doesn't need any update, we don't want to do anything. > > In order to check if the tick needs an update, we compare it against the > clockevent device deadline. Now that's a problem because the clockevent > device is at a lower level than the tick itself if it is implemented > on top of hrtimer. > > Every hrtimer share this clockevent device. So comparing the next tick > deadline against the clockevent device deadline is wrong because the > device may be programmed for another hrtimer whose deadline collides > with the tick. As a result we may end up not reprogramming the tick > accidentally. > > In a worst case scenario under full dynticks mode, the tick stops firing > as it is supposed to every 1hz, leaving /proc/stat stalled: > > Task in a full dynticks CPU > ---------------------------- > > * hrtimer A is queued 2 seconds ahead > * the tick is stopped, scheduled 1 second ahead > * tick fires 1 second later > * on tick exit, nohz schedules the tick 1 second ahead but sees > the clockevent device is already programmed to that deadline, > fooled by hrtimer A, the tick isn't rescheduled. > * hrtimer A is cancelled before its deadline > * tick never fires again until an interrupt happens... > > In order to fix this, store the next tick deadline to the tick_sched > local structure and reuse that value later to check whether we need to > reprogram the clock after an interrupt. > > On the other hand, ts->sleep_length still wants to know about the next > clock event and not just the tick, so we want to improve the related > comment to avoid confusion. > > Acked-by: Rik van Riel > Reviewed-by: Wanpeng Li > Reported-by: James Hartsock > Cc: Ingo Molnar > Cc: Rik van Riel Acked-by: Peter Zijlstra (Intel)