linux-kernel - Re: [PATCH V2] kernel/watchdog: fix spurious hard lockups

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Date:   Wed, 21 Jun 2017 13:40:23 -0400
From:   Prarit Bhargava <prarit@...hat.com>
To:     "Liang, Kan" <kan.liang@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dzickus@...hat.com" <dzickus@...hat.com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "babu.moger@...cle.com" <babu.moger@...cle.com>,
        "atomlin@...hat.com" <atomlin@...hat.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "eranian@...gle.com" <eranian@...gle.com>,
        "acme@...hat.com" <acme@...hat.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH V2] kernel/watchdog: fix spurious hard lockups



On 06/21/2017 11:47 AM, Liang, Kan wrote:
> 
> 
>> On Wed, 21 Jun 2017, kan.liang@...el.com 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?
> 

cpufreq_quick_get_max()

but iff cpufreq subsystem is initialized.  O/w 0 is returned for the freq.

Quick test shows the correct turbo max of 3700000 on my 2000000 (2.00GHz)
system.

P.

> 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
>