From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760401Ab2CNUjk (ORCPT ); Wed, 14 Mar 2012 16:39:40 -0400 Received: from mx1.redhat.com ([209.132.183.28]:4535 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753875Ab2CNUjj (ORCPT ); Wed, 14 Mar 2012 16:39:39 -0400 From: Don Zickus To: LKML Cc: Michal Hocko , Ingo Molnar , Peter Zijlstra , Andrew Morton , Mandeep Singh Baines , Don Zickus Subject: [PATCH] watchdog: Make sure the watchdog thread gets CPU on loaded system Date: Wed, 14 Mar 2012 16:38:45 -0400 Message-Id: <1331757525-5755-1-git-send-email-dzickus@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Michal Hocko If the system is loaded while hotplugging a CPU we might end up with a bogus hardlockup detection. This has been seen during LTP pounder test executed in parallel with hotplug test. The main problem is that enable_watchdog (called when CPU is brought up) registers perf event which periodically checks per-cpu counter (hrtimer_interrupts), updated from a hrtimer callback, but the hrtimer is fired from the kernel thread. This means that while we already do check for the hard lockup the kernel thread might be sitting on the runqueue with zillions of tasks so there is nobody to update the value we rely on and so we KABOOM. Let's fix this by boosting the watchdog thread priority before we wake it up rather than when it's already running. This still doesn't handle a case where we have the same amount of high prio FIFO tasks but that doesn't seem to be common. The current implementation doesn't handle that case anyway so this is not worse at least. Unfortunately, we cannot start perf counter from the watchdog thread because we could miss a real lock up and also we cannot start the hrtimer watchdog_enable because we there is no way (at least I don't know any) to start a hrtimer from a different CPU. [fix compile issue with param -dcz] Cc: Ingo Molnar Cc: Peter Zijlstra Cc: Andrew Morton Cc: Mandeep Singh Baines Signed-off-by: Michal Hocko Signed-off-by: Don Zickus --- kernel/watchdog.c | 7 +++---- 1 files changed, 3 insertions(+), 4 deletions(-) diff --git a/kernel/watchdog.c b/kernel/watchdog.c index d117262..6618cde 100644 --- a/kernel/watchdog.c +++ b/kernel/watchdog.c @@ -321,11 +321,9 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer) */ static int watchdog(void *unused) { - struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 }; + struct sched_param param = { .sched_priority = 0 }; struct hrtimer *hrtimer = &__raw_get_cpu_var(watchdog_hrtimer); - sched_setscheduler(current, SCHED_FIFO, ¶m); - /* initialize timestamp */ __touch_watchdog(); @@ -350,7 +348,6 @@ static int watchdog(void *unused) set_current_state(TASK_INTERRUPTIBLE); } __set_current_state(TASK_RUNNING); - param.sched_priority = 0; sched_setscheduler(current, SCHED_NORMAL, ¶m); return 0; } @@ -439,6 +436,7 @@ static int watchdog_enable(int cpu) /* create the watchdog thread */ if (!p) { + struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 }; p = kthread_create_on_node(watchdog, NULL, cpu_to_node(cpu), "watchdog/%d", cpu); if (IS_ERR(p)) { printk(KERN_ERR "softlockup watchdog for %i failed\n", cpu); @@ -450,6 +448,7 @@ static int watchdog_enable(int cpu) } goto out; } + sched_setscheduler(p, SCHED_FIFO, ¶m); kthread_bind(p, cpu); per_cpu(watchdog_touch_ts, cpu) = 0; per_cpu(softlockup_watchdog, cpu) = p; -- 1.7.7.6