Re: RFC: kvmclock / tsc server side fix

[Zachary Amsden <zamsden@redhat.com>]

On 05/18/2010 04:08 AM, Glauber Costa wrote:
> On Mon, May 17, 2010 at 09:38:30AM -1000, Zachary Amsden wrote:
>    
>> On 05/17/2010 05:36 AM, Glauber Costa wrote:
>>      
>>> On Fri, May 14, 2010 at 04:07:43PM -1000, Zachary Amsden wrote:
>>>        
>>>> I believe this fixes the root cause of the kvmclock warp.  It's
>>>> quite a plausible phenomenon, and explains why it was so easy to
>>>> produce.
>>>>
>>>>          
>>> You mean this is the case for both SMP and UP, or just UP as we talked
>>> before?
>>>        
>> It's possible on both SMP and UP, guest and host.  It is impossible
>> on UP host unless special circumstances come into play (one of my
>> patches created these circumstances).
>>
>>      
>>> I don't get the role of upscale in your patch. Frequency changes are
>>> already handled by the cpufreq notifier.
>>>        
>> The only purpose of upscale is to downscale the measurement of delta
>> used for counting stats if CPU frequency was raised since last
>> observed.  This is because moving to a faster TSC rate means we
>> might have counted some cycles at the wrong rate while the rate was
>> in transition.  It doesn't much matter, as the delta for which
>> "overrun" is logged was computed wrong anyway.
>>      
> mind sending the stats part in a separate patch?
>    

Yeah, part of this was badly broken.
>    
>> I'll clean up my patches and resend as a series today.
>>
>> Zach
>>      

Or today.  Nasty bug.