Re: [patch 0/2] tsc/adjust: Cure suspend/resume issues and prevent TSC deadline timer irq storm

[Roland Scheidegger]

Am 14.12.2016 um 22:40 schrieb Thomas Gleixner:
> On Wed, 14 Dec 2016, Thomas Gleixner wrote:
>> Positive space, results in timer not firing anymore - at least not in a
>> time frame you are willing to wait for.
>>
>>      0x0000 0000 8000 0000
>>
>> Negative space, results in an interrupt storm.
>>
>>      0xffff ffff 0000 0000
>>      0xffff fffe 0000 0000
>>      0xffff fffd 0000 0000
>>      0xffff fffc 0000 0000
>>      0xffff fffb 0000 0000
>>      ....
>>
>> These points are independent of the underlying counter value (cold boot,
>> warm boot) and even reproduce after hours of power on reliably.
>>
>> And looking at the values makes me wonder about 32bit vs. 64bit wreckage
>> combined with sign expansion done wrong. Im really impressed!
> 
> And the whole mess stems from the fact that the deadline is not as one
> would expect simply compared against the sum of the counter and the adjust
> MSR.
Why would it be compared against the sum? As far as I can tell the adjust
value should never come into play when using deadline timer (other than
indirectly because the TSC would change).
(And I'd guess better avoid an armed deadline timer while changing
TSC_ADJ...)

In any case, I've tested the two patches on top of x86-timers and they
work just fine - all TSC_ADJ values get set to zero both on boot and
resume, no lockups, and tsc clocksource active, with some whining in the
log of course :-).
So,
Tested-by: Roland Scheidegger <rscheidegger_lists@xxxxxxxxxx>

> No, they subtract the adjust value from the MSR when you write the deadline
> and latch the result to compare it against the counter.
> 
> So when the following happens:
> 
>    ADJUST	= 0
>    RDTSC	= 10000000 
>    DEADLINE	= 11000000
> 
>    ADJUST	=  1000000
> 
>    INTERRUPT
>    RDTSC	= 12000000
> 
>    DEADLINE	= 13000000
> 
>    ADJUST	=        0
> 
>    INTERRUPT
>    RDTSC	= 12000000
> 
> So depending on the direction of the adjustment the timer fires late or
> early.
> 
> Combined with that math wreckage this is a complete disaster. And of course
> nothing is documented anywhere and the SDM is outright wrong:
> 
> 10.5.4.1 TSC-Deadline Mode
> 
>   The processor generates a timer interrupt when the value of time-stamp
>   counter is greater than or equal to that of IA32_TSC_DEADLINE. It then
>   disarms the timer and clear the IA32_TSC_DEADLINE MSR. (Both the time-stamp
>   counter and the IA32_TSC_DEADLINE MSR are 64-bit unsigned integers.)
> 
> See the example above. 1200000 is neither equal nor greater than 1300000, at
> least not in my universe.
> 
> I serioulsy doubt that Intel manages it to design at least ONE functional
> non broken timer before I retire.
> 
> Thanks,
> 
> 	tglx
>