linux-kernel - Re: [PATCH 1/8] [watchdog] combine nmi

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Message-ID: <20100428202810.GR15159@redhat.com>
Date:	Wed, 28 Apr 2010 16:28:10 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	mingo@...e.hu, peterz@...radead.org, gorcunov@...il.com,
	aris@...hat.com, linux-kernel@...r.kernel.org,
	randy.dunlap@...cle.com
Subject: Re: [PATCH 1/8] [watchdog] combine nmi_watchdog and softlockup

On Wed, Apr 28, 2010 at 02:36:54PM +0200, Frederic Weisbecker wrote:
> On Fri, Apr 23, 2010 at 12:13:29PM -0400, Don Zickus wrote:
> > +void watchdog_overflow_callback(struct perf_event *event, int nmi,
> > +		 struct perf_sample_data *data,
> > +		 struct pt_regs *regs)
> > +{
> > +	int this_cpu = smp_processor_id();
> > +	unsigned long touch_ts = per_cpu(watchdog_touch_ts, this_cpu);
> > +	char warn = __get_cpu_var(watchdog_warn);
> > +
> > +	if (touch_ts == 0) {
> > +		__touch_watchdog();
> > +		return;
> > +	}
> > +
> > +	/* check for a hardlockup
> > +	 * This is done by making sure our timer interrupt
> > +	 * is incrementing.  The timer interrupt should have
> > +	 * fired multiple times before we overflow'd.  If it hasn't
> > +	 * then this is a good indication the cpu is stuck
> > +	 */
> > +	if (is_hardlockup(this_cpu)) {
> > +		/* only print hardlockups once */
> > +		if (warn & HARDLOCKUP)
> > +			return;
> > +
> > +		if (hardlockup_panic)
> > +			panic("Watchdog detected hard LOCKUP on cpu %d", this_cpu);
> > +		else
> > +			WARN(1, "Watchdog detected hard LOCKUP on cpu %d", this_cpu);
> > +
> > +		__get_cpu_var(watchdog_warn) = warn | HARDLOCKUP;
> > +		return;
> > +	}
> > +
> > +	__get_cpu_var(watchdog_warn) = warn & ~HARDLOCKUP;
> > +	return;
> > +}
> [...]
> > +static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
> > +{
> > +	int this_cpu = smp_processor_id();
> > +	unsigned long touch_ts = __get_cpu_var(watchdog_touch_ts);
> > +	char warn = __get_cpu_var(watchdog_warn);
> > +	struct pt_regs *regs = get_irq_regs();
> > +	int duration;
> > +
> > +	/* kick the hardlockup detector */
> > +	watchdog_interrupt_count();
> > +
> > +	/* kick the softlockup detector */
> > +	wake_up_process(__get_cpu_var(softlockup_watchdog));
> > +
> > +	/* .. and repeat */
> > +	hrtimer_forward_now(hrtimer, ns_to_ktime(get_sample_period()));
> > +
> > +	if (touch_ts == 0) {
> > +		__touch_watchdog();
> > +		return HRTIMER_RESTART;
> > +	}
> > +
> > +	/* check for a softlockup
> > +	 * This is done by making sure a high priority task is
> > +	 * being scheduled.  The task touches the watchdog to
> > +	 * indicate it is getting cpu time.  If it hasn't then
> > +	 * this is a good indication some task is hogging the cpu
> > +	 */
> > +	duration = is_softlockup(touch_ts, this_cpu);
> > +	if (unlikely(duration)) {
> > +		/* only warn once */
> > +		if (warn & SOFTLOCKUP)
> > +			return HRTIMER_RESTART;
> > +
> > +		printk(KERN_ERR "BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n",
> > +			this_cpu, duration,
> > +			current->comm, task_pid_nr(current));
> > +		print_modules();
> > +		print_irqtrace_events(current);
> > +		if (regs)
> > +			show_regs(regs);
> > +		else
> > +			dump_stack();
> > +
> > +		if (softlockup_panic)
> > +			panic("softlockup: hung tasks");
> > +		__get_cpu_var(watchdog_warn) = warn | SOFTLOCKUP;
> > +	} else
> > +		__get_cpu_var(watchdog_warn) = warn & ~SOFTLOCKUP;
> 
> 
> Note these watchdog_warn modifications are racy against the same that
> happens with HARDLOCKUP. You might clear what did the nmi.
> 
> The race is harmless enough that we don't care much I think, but that's
> why it would have make sense to separate watchdog_warn tracking space
> between both.

Heh.  Good point.  I'll respin.

Cheers,
Don

> 
> 
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