From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752047AbaH2B1P (ORCPT ); Thu, 28 Aug 2014 21:27:15 -0400 Received: from mx1.redhat.com ([209.132.183.28]:40867 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751358AbaH2B1O (ORCPT ); Thu, 28 Aug 2014 21:27:14 -0400 Date: Thu, 28 Aug 2014 21:27:05 -0400 From: Don Zickus To: Andrew Morton Cc: LKML , Ingo Molnar , chai wen Subject: Re: [PATCH] softlockup: Make detector be aware of task switch of processes hogging cpu Message-ID: <20140829012705.GC49576@redhat.com> References: <1409201544-82371-1-git-send-email-dzickus@redhat.com> <20140828160723.b174b4e03f2854d0a5146199@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140828160723.b174b4e03f2854d0a5146199@linux-foundation.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 28, 2014 at 04:07:23PM -0700, Andrew Morton wrote: > On Thu, 28 Aug 2014 00:52:24 -0400 Don Zickus wrote: > > > From: chai wen > > > > For now, soft lockup detector warns once for each case of process softlockup. > > But the thread 'watchdog/n' may not always get the cpu at the time slot between > > the task switch of two processes hogging that cpu to reset soft_watchdog_warn. > > > > An example would be two processes hogging the cpu. Process A causes the > > softlockup warning and is killed manually by a user. Process B immediately > > becomes the new process hogging the cpu preventing the softlockup code from > > resetting the soft_watchdog_warn variable. > > > > This case is a false negative of "warn only once for a process", as there may > > be a different process that is going to hog the cpu. Resolve this by > > saving/checking the task pointer of the hogging process and use that to reset > > soft_watchdog_warn too. > > > > OK, this should address the PID uniqueness issue which Ingo identified. > > > --- a/kernel/watchdog.c > > +++ b/kernel/watchdog.c > > @@ -47,6 +47,7 @@ static DEFINE_PER_CPU(bool, softlockup_touch_sync); > > static DEFINE_PER_CPU(bool, soft_watchdog_warn); > > static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts); > > static DEFINE_PER_CPU(unsigned long, soft_lockup_hrtimer_cnt); > > +static DEFINE_PER_CPU(struct task_struct *, softlockup_task_ptr_saved); > > #ifdef CONFIG_HARDLOCKUP_DETECTOR > > static DEFINE_PER_CPU(bool, hard_watchdog_warn); > > static DEFINE_PER_CPU(bool, watchdog_nmi_touch); > > @@ -331,8 +332,20 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer) > > return HRTIMER_RESTART; > > > > /* only warn once */ > > - if (__this_cpu_read(soft_watchdog_warn) == true) > > + if (__this_cpu_read(soft_watchdog_warn) == true) { > > + /* > > + * Handle the case where multiple processes are > > + * causing softlockups but the duration is small > > + * enough, the softlockup detector can not reset > > + * itself in time. Use task pointers to detect this. > > + */ > > This comment is rather hard to follow ("the duration" of what?). Can > you think of some words which are a bit more complete/clear? Agreed. Does this work better? " /* * When multiple processes are causing softlockups * the softlockup detector only warns on the first * one because the code relies on a full quiet cycle * to re-arm. The second process prevents the * quiet cycle and never gets reported. Use task * pointers to detect this. */ Cheers, Don