Re: [Xen-devel] [PATCH 1/8] x86/time: improve cross-CPU clock monotonicity (and more)

[Joao Martins <joao.m.martins@xxxxxxxxxx>]

On 06/21/2016 01:28 PM, Jan Beulich wrote:
>>>> On 21.06.16 at 14:05, <joao.m.martins@xxxxxxxxxx> wrote:
> 
>>
>> On 06/17/2016 08:32 AM, Jan Beulich wrote:
>>>>>> On 16.06.16 at 22:27, <joao.m.martins@xxxxxxxxxx> wrote:
>>>>> I.e. my plan was, once the backwards moves are small enough, to maybe
>>>>> indeed compensate them by maintaining a global variable tracking
>>>>> the most recently returned value. There are issues with such an
>>>>> approach too, though: HT effects can result in one hyperthread
>>>>> making it just past that check of the global, then hardware
>>>>> switching to the other hyperthread, NOW() producing a slightly
>>>>> larger value there, and hardware switching back to the first
>>>>> hyperthread only after the second one consumed the result of
>>>>> NOW(). Dario's use would be unaffected by this aiui, as his NOW()
>>>>> invocations are globally serialized through a spinlock, but arbitrary
>>>>> NOW() invocations on two hyperthreads can't be made such that
>>>>> the invoking party can be guaranteed to see strictly montonic
>>>>> values.
>>>>>
>>>>> And btw., similar considerations apply for two fully independent
>>>>> CPUs, if one runs at a much higher P-state than the other (i.e.
>>>>> the faster one could overtake the slower one between the
>>>>> montonicity check in NOW() and the callers consuming the returned
>>>>> values). So in the end I'm not sure it's worth the performance hit
>>>>> such a global montonicity check would incur, and therefore I didn't
>>>>> make a respective patch part of this series.
>>>>>
>>>>
>>>> Hm, guests pvclock should have faced similar issues too as their
>>>> local stamps for each vcpu diverge. Linux commit 489fb49 ("x86, paravirt: 
>>>> Add a
>>>> global synchronization point for pvclock") depicts a fix to similar 
>>>> situations to the
>>>> scenarios you just described - which lead to have a global variable to 
>>>> keep 
>>>> track of
>>>> most recent timestamp. One important chunk of that commit is pasted below 
>>>> for
>>>> convenience:
>>>>
>>>> --
>>>> /*
>>>>  * Assumption here is that last_value, a global accumulator, always goes
>>>>  * forward. If we are less than that, we should not be much smaller.
>>>>  * We assume there is an error marging we're inside, and then the 
>>>> correction
>>>>  * does not sacrifice accuracy.
>>>>  *
>>>>  * For reads: global may have changed between test and return,
>>>>  * but this means someone else updated poked the clock at a later time.
>>>>  * We just need to make sure we are not seeing a backwards event.
>>>>  *
>>>>  * For updates: last_value = ret is not enough, since two vcpus could be
>>>>  * updating at the same time, and one of them could be slightly behind,
>>>>  * making the assumption that last_value always go forward fail to hold.
>>>>  */
>>>>  last = atomic64_read(&last_value);
>>>>  do {
>>>>      if (ret < last)
>>>>          return last;
>>>>      last = atomic64_cmpxchg(&last_value, last, ret);
>>>>  } while (unlikely(last != ret));
>>>> --
>>>
>>> Meaning they decided it's worth the overhead. But (having read
>>> through the entire description) they don't even discuss whether this
>>> indeed eliminates _all_ apparent backward moves due to effects
>>> like the ones named above.
>>>
>>> Plus, the contention they're facing is limited to a single VM, i.e. likely
>>> much more narrow than that on an entire physical system. So for
>>> us to do the same in the hypervisor, quite a bit more of win would
>>> be needed to outweigh the cost.
>>>
>> Experimental details look very unclear too - likely running the time
>> warp test for 5 days would get some of these cases cleared out. But
>> as you say it should be much more narrow that of an entire system.
>>
>> BTW It was implicit in the discussion but my apologies for not
>> formally/explicitly stating. So FWIW:
>>
>> Tested-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
> 
> Thanks, but this ...
> 
>> This series is certainly a way forward into improving cross-CPU monotonicity,
>> and I am seeing indeed less occurrences of time going backwards on my 
>> systems.
> 
> ... leaves me guessing whether the above was meant for just this
> patch, or the entire series.
> 
Ah sorry, a little ambiguous on my end - It is meant for the entire series.

Joao

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