From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752009AbaHRSCU (ORCPT ); Mon, 18 Aug 2014 14:02:20 -0400 Received: from mail-wi0-f178.google.com ([209.85.212.178]:49659 "EHLO mail-wi0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751676AbaHRSCS (ORCPT ); Mon, 18 Aug 2014 14:02:18 -0400 Date: Mon, 18 Aug 2014 20:01:58 +0200 From: Ingo Molnar To: Don Zickus Cc: akpm@linux-foundation.org, kvm@vger.kernel.org, pbonzini@redhat.com, mingo@redhat.com, LKML , chai wen Subject: Re: [PATCH 2/5] softlockup: make detector be aware of task switch of processes hogging cpu Message-ID: <20140818180158.GA4540@gmail.com> References: <1407768567-171794-1-git-send-email-dzickus@redhat.com> <1407768567-171794-3-git-send-email-dzickus@redhat.com> <20140818090319.GA25495@gmail.com> <20140818150658.GQ49576@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140818150658.GQ49576@redhat.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Don Zickus wrote: > On Mon, Aug 18, 2014 at 11:03:19AM +0200, Ingo Molnar wrote: > > * 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 pid of the hogging process and use that to reset > > > soft_watchdog_warn too. > > > > > > Signed-off-by: chai wen > > > [modified the comment and changelog to be more specific] > > > Signed-off-by: Don Zickus > > > --- > > > kernel/watchdog.c | 20 ++++++++++++++++++-- > > > 1 files changed, 18 insertions(+), 2 deletions(-) > > > > > > diff --git a/kernel/watchdog.c b/kernel/watchdog.c > > > index 4c2e11c..6d0a891 100644 > > > --- a/kernel/watchdog.c > > > +++ b/kernel/watchdog.c > > > @@ -42,6 +42,7 @@ 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(pid_t, softlockup_warn_pid_saved); > > > #ifdef CONFIG_HARDLOCKUP_DETECTOR > > > static DEFINE_PER_CPU(bool, hard_watchdog_warn); > > > static DEFINE_PER_CPU(bool, watchdog_nmi_touch); > > > @@ -317,6 +318,8 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer) > > > */ > > > duration = is_softlockup(touch_ts); > > > if (unlikely(duration)) { > > > + pid_t pid = task_pid_nr(current); > > > + > > > /* > > > * If a virtual machine is stopped by the host it can look to > > > * the watchdog like a soft lockup, check to see if the host > > > @@ -326,8 +329,20 @@ 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 pids to detect this. > > > + */ > > > + if (__this_cpu_read(softlockup_warn_pid_saved) != pid) { > > > > So I agree with the motivation of this improvement, but is this > > implementation namespace-safe? > > What namespace are you worried about colliding with? I thought > softlockup_ would provide the safety?? Maybe I am missing something > obvious. :-( I meant PID namespaces - a PID in itself isn't guaranteed to be unique across the system. Thanks, Ingo