Re: [PATCH] cpufreq, Fix overflow in busy_scaled due to long delay

[Prarit Bhargava]

On 06/11/2015 10:51 AM, Doug Smythies wrote:
> 
> On 2015.06.10 16:46 Rafael J. Wysocki wrote:
>> On Wednesday, June 10, 2015 09:18:45 AM Prarit Bhargava wrote:
>>> I looked into switching to div64_s64() instead of the 32-bit version in
>>> div_fp(), however, this would result in sample_ratio and core_busy returning
>>> 0 which is something we don't want.
> 
> ???
> Due to a great many overflow related issues, div_fp() was changed to div64_s64()
> a long time ago.

Doug,

Nope -- in a linux.git tree (up-to-date as of 7:00AM ET this AM)

static inline int32_t div_fp(int32_t x, int32_t y)
{
        return div_s64((int64_t)x << FRAC_BITS, y);
}

If we do want this to be div64_s64, I can make that change, however, I feel that
a long delay like this should be ignored in the performance calculations in the
driver and that's why I chose to go the direction I did.

P.

> 
> I have not found the actual commit to reference, but it was about a year ago.
> And the math in general was all changed to 64 bit, over a few commits.
> 
>>
>> P.
>>
>> ---8<---
>>
>> The kernel may delay interrupts for a long time which can result in timers
>> being delayed.  If this occurs the intel_pstate driver will crash with
>> a divide by zero error:
> 
> More recent versions will not crash.
> Long timer delays are extremely common, and this is a fundamental flaw
> in the duration method. Patch sets have been submitting dealing with this,
> and other, issues.
> 
>>>
>>> which results in the time between samples = last_sample_time - sample.time
>>> = 4063149215234118 - 4063132438017305 = 16777216813 which is 16.777 seconds.
> 
> I have never seen anything over 4 seconds before, and I study this stuff
> (with respect to the intel_pstate driver operation) a lot. Due to help
> from others, I have data from a variety of processors.
> 4 seconds not unusual, even under load.
> 
>>>
>>> The duration between reads of the APERF and MPERF registers overflowed a s32
>>> sized integer in intel_pstate_get_scaled_busy()'s call to div_fp().  The result
>>> is that int_tofp(duration_us) == 0, and the kernel attempts to divide by 0.
>>>
>>> While the kernel shouldn't be delaying for a long time, it can and does
>>> happen, and the intel_pstate driver should not panic in this situation.  This
>>> patch checks for an overflow and ignores the current calculation cycle by
>>> returning -EINVAL.  Since intel_pstate_sample() has been called, subsequent
>>> timer function calls will then again pick up the correct calculations and the
>>> system will continue functioning properly.
> 
> That would run the risk that the correct calculation would never be done.
> It is fairly easy (I do it all the time) to create a scenario where
> there is high load on a CPU, but also a very very high duration value,
> for each and every duration. (and O.K., in that scenario the calculation is always
> wrong anyhow, due to the long duration check engaging.)
> 
> 
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