Re: [Xen-devel] [PATCH v5] new config option vtsc_tolerance_khz to avoid TSC emulation

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 03/28/2018 05:52 PM, Wei Liu wrote:
> On Wed, Mar 28, 2018 at 05:49:28PM +0100, George Dunlap wrote:
>> On 03/28/2018 05:27 PM, Wei Liu wrote:
>>> On Tue, Mar 27, 2018 at 11:26:55AM +0200, Olaf Hering wrote:
>>>> Add an option to control when vTSC emulation will be activated for a
>>>> domU with tsc_mode=default. Without such option each TSC access from
>>>> domU will be emulated, which causes a significant perfomance drop for
>>>> workloads that make use of rdtsc.
>>>>
>>>> One option to avoid the TSC option is to run domUs with tsc_mode=native.
>>>> This has the drawback that migrating a domU from a "2.3GHz" class host
>>>> to a "2.4GHz" class host may change the rate at wich the TSC counter
>>>> increases, the domU may not be prepared for that.
>>>>
>>>> With the new option the host admin can decide how a domU should behave
>>>> when it is migrated across systems of the same class. Since there is
>>>> always some jitter when Xen calibrates the cpu_khz value, all hosts of
>>>> the same class will most likely have slightly different values. As a
>>>> result vTSC emulation is unavoidable. Data collected during the incident
>>>> which triggered this change showed a jitter of up to 200 KHz across
>>>> systems of the same class.
>>>>
>>>> Existing padding fields are reused to store vtsc_khz_tolerance as u16.
>>>>
>>> [...]
>>>> index 2c1a6e1422..0b36265e4f 100644
>>>> --- a/docs/man/xl.cfg.pod.5.in
>>>> +++ b/docs/man/xl.cfg.pod.5.in
>>>> @@ -1891,6 +1891,16 @@ determined in a similar way to that of B<default> 
>>>> TSC mode.
>>>>  
>>>>  Please see B<xen-tscmode(7)> for more information on this option.
>>>>  
>>>> +=item B<vtsc_tolerance_khz="KHZ">
>>>> +
>>>> +B<(x86 only, relevant only for tsc_mode=default)>
>>>> +When a domU is started, the CPU frequency of the host is used by the domU 
>>>> for
>>>> +TSC related time measurement. Once the domU is either migrated or
>>>> +saved/restored on another host that CPU frequency has to be emulated to 
>>>> avoid
>>>> +timedrift. To avoid the performance penalty of the TSC emulation, allow a
>>>> +certain amount of jitter of the measured CPU frequency on the hosts the 
>>>> domU
>>>> +is supposed to run on.
>>>
>>> "Default value is 0, i.e. no tolerance".
>>>
>>> Can we get an agreement on whether this idea the right approach in
>>> general before I do detail review?
>>
>> I'm not super-familiar with this area, but looking from the outside I
>> think Olaf's approach (having a "tolerance" for equivalence) makes
>> sense.  Can't comment on what kind of a hypercall it should be.
>>
> 
> I have a rough idea how the code should look like -- the patch in its
> current form looks mostly OK, but not sure if this approach in general
> is future proof.

What do you mean?  And do you have an idea for a better way to solve the
problem?  Or do you think it's not a very big problem?

 -George

 -George

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