linux-kernel - Re: [watchdog] combine nmi

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Date:	Tue, 6 Apr 2010 19:31:15 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Don Zickus <dzickus@...hat.com>, mingo@...e.hu,
	peterz@...radead.org, aris@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [watchdog] combine nmi_watchdog and softlockup

On Tue, Apr 06, 2010 at 04:13:30PM +0200, Frederic Weisbecker wrote:
[...]
> > +static int watchdog_enable(int cpu)
> > +{
> > +	struct perf_event_attr *wd_attr;
> > +	struct perf_event *event = per_cpu(watchdog_ev, cpu);
> > +	struct task_struct *p = per_cpu(softlockup_watchdog, cpu);
> > +
> > +	/* is it already setup and enabled? */
> > +	if (event && event->state > PERF_EVENT_STATE_OFF)
> > +		goto out;
> > +
> > +	/* it is setup but not enabled */
> > +	if (event != NULL)
> > +		goto out_enable;
> > +
> > +	/* Try to register using hardware perf events first */
> > +	wd_attr = &wd_hw_attr;
> > +	wd_attr->sample_period = hw_nmi_get_sample_period();
> > +	event = perf_event_create_kernel_counter(wd_attr, cpu, -1, watchdog_overflow_callback);
> > +	if (!IS_ERR(event)) {
> > +		printk(KERN_INFO "NMI watchdog enabled, takes one hw-pmu counter.\n");
> > +		goto out_save;
> > +	}
> > +
> > +	/* hardware doesn't exist or not supported, fallback to software events */
> > +	printk(KERN_INFO "NMI watchdog: hardware not available, trying software events\n");
> > +	wd_attr = &wd_sw_attr;
> > +	wd_attr->sample_period = softlockup_thresh * NSEC_PER_SEC;
> > +	event = perf_event_create_kernel_counter(wd_attr, cpu, -1, watchdog_overflow_callback);
> 
> I fear the cpu clock is not going to help you detecting any hard lockups.
> If you're stuck in an interrupt or an irq disabled loop, your cpu clock is
> not going to fire.
>

I guess it's not supposed to. For such cases only nmi irqs may help for which
the perf events are there (/me need to check if we program apic timer for anything
like that). But it should help for other deadlocks. Or I miss something?

	-- Cyrill
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