RE: [PATCH V2] kernel/watchdog: fix spurious hard lockups

[Liang, Kan]

> On Wed, 21 Jun 2017, kan.liang@xxxxxxxxx wrote:
> >
> >  #ifdef CONFIG_HARDLOCKUP_DETECTOR
> > +/*
> > + * The NMI watchdog relies on PERF_COUNT_HW_CPU_CYCLES event,
> which
> > + * can tick faster than the measured CPU Frequency due to Turbo mode.
> > + * That can lead to spurious timeouts.
> > + * To workaround the issue, extending the period by 3 times.
> > + */
> >  u64 hw_nmi_get_sample_period(int watchdog_thresh)  {
> > -	return (u64)(cpu_khz) * 1000 * watchdog_thresh;
> > +	return (u64)(cpu_khz) * 1000 * watchdog_thresh * 3;
> 
> The maximum turbo frequency of any given machine can be retrieved.

The maximum turbo frequency is determined by the model of processor.
I'm not sure if there is a generic way to get the maximum turbo frequency.
Is there?

Thanks,
Kan

> 
> So why don't you simply take that ratio into account and apply it for the
> machines which have those insane turbo loaders? That's not a huge effort,
> can be easily backported and does not inflict this unconditially.
> 
> So what you want is:
> 
> 	return get_max_turbo_khz() * 1000 * watchdog_thresh;
> 
> Where get_max_turbo_khz() by default returns cpu_khz for non turbo
> motors.
> 
> And instead of silently doing this it should emit a info into dmesg:
> 
>    	u64 period, max_khz = get_max_turbo_khz();
> 	static int once;
> 
> 	period = max_khz * 1000 * watchdog_thresh;
> 
> 	if (max_khz != cpu_khz && !once) {
> 		unsigned int msec = period / cpu_khz;
> 
> 		once = 1;
> 		pr_info("Adjusted watchdog threshold to %u.%04u sec\n",
> 			msec / 1000, msec % 1000);
> 	}
> 
> 	return period;
> 
> Hmm?
> 
> Thanks,
> 
> 	tglx