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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Mar 28, 2018 at 06:07:53PM +0100, George Dunlap wrote:
> 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?

Whether introducing this special parameter is future proof, i.e. it
won't come back and bite us big time. I don't have better idea on
solving this problem.

Wei.

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