linux-kernel - Re: [PATCH 3/4] nmi_watchdog: fallback to software events when no hardware pmu detected

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Date:	Mon, 15 Feb 2010 10:38:37 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, peterz@...radead.org,
	gorcunov@...il.com, aris@...hat.com
Subject: Re: [PATCH 3/4] nmi_watchdog: fallback to software events when no
 hardware pmu detected

On Mon, Feb 15, 2010 at 11:33:41AM +1100, Paul Mackerras wrote:
> On Fri, Feb 12, 2010 at 05:19:20PM -0500, Don Zickus wrote:
> 
> > diff --git a/kernel/nmi_watchdog.c b/kernel/nmi_watchdog.c
> > index 73c1954..4f23505 100644
> > --- a/kernel/nmi_watchdog.c
> > +++ b/kernel/nmi_watchdog.c
> > @@ -166,8 +166,12 @@ cpu_callback(struct notifier_block *nfb, unsigned long action, void *hcpu)
> >  		wd_attr.sample_period = hw_nmi_get_sample_period();
> >  		event = perf_event_create_kernel_counter(&wd_attr, hotcpu, -1, wd_overflow);
> >  		if (IS_ERR(event)) {
> > -			printk(KERN_ERR "nmi watchdog failed to create perf event on %i: %p\n", hotcpu, event);
> > -			return NOTIFY_BAD;
> > +			wd_attr.type = PERF_TYPE_SOFTWARE;
> > +			event = perf_event_create_kernel_counter(&wd_attr, hotcpu, -1, wd_overflow);
> 
> Here you don't explicitly set wd_attr.config or wd_attr.sample_period
> for the software event.  So PERF_COUNT_HW_CPU_CYCLES (which is 0)
> becomes PERF_COUNT_SW_CPU_CLOCK (also 0).  Which is either a happy
> accident or really really subtle. :)  I suggest you either set
> wd_attr.config or add a comment pointing out that you are using the
> fact that PERF_COUNT_HW_CPU_CYCLES == PERF_COUNT_SW_CPU_CLOCK.
> 
> Also you don't explicitly set wd_attr.sample_period, so it will be
> whatever hw_nmi_get_sample_period() returned, except now measured in
> nanoseconds rather than (presumably) cpu clock cycles.  Are you aiming
> for 1 interrupt per second?  If so you should set .sample_period to
> NSEC_PER_SEC.

Hmm, good points, I'll work on that and add it to my next round of
changes.

Thanks,
Don
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