Re: [PATCH v2 2/3] x86/time: adjust time recording time_calibration_tsc_rendezvous()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Feb 08, 2021 at 11:56:01AM +0100, Jan Beulich wrote:
> On 05.02.2021 17:15, Roger Pau Monné wrote:
> > On Mon, Feb 01, 2021 at 01:43:04PM +0100, Jan Beulich wrote:
> >> The (stime,tsc) tuple is the basis for extrapolation by get_s_time().
> >> Therefore the two better get taken as close to one another as possible.
> >> This means two things: First, reading platform time is too early when
> >> done on the first iteration. The closest we can get is on the last
> >> iteration, immediately before telling other CPUs to write their TSCs
> >> (and then also writing CPU0's). While at the first glance it may seem
> >> not overly relevant when exactly platform time is read (when assuming
> >> that only stime is ever relevant anywhere, and hence the association
> >> with the precise TSC values is of lower interest), both CPU frequency
> >> changes and the effects of SMT make it unpredictable (between individual
> >> rendezvous instances) how long the loop iterations will take. This will
> >> in turn lead to higher an error than neccesary in how close to linear
> >> stime movement we can get.
> >>
> >> Second, re-reading the TSC for local recording is increasing the overall
> >> error as well, when we already know a more precise value - the one just
> >> written.
> >>
> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> > 
> > Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> Thanks.
> 
> > I've been thinking this all seems doomed when Xen runs in a virtualized
> > environment, and should likely be disabled. There's no point on trying
> > to sync the TSC over multiple vCPUs as the scheduling delay between
> > them will likely skew any calculations.
> 
> We may want to consider to force the equivalent of
> "clocksource=tsc" in that case. Otoh a well behaved hypervisor
> underneath shouldn't lead to us finding a need to clear
> TSC_RELIABLE, at which point this logic wouldn't get engaged
> in the first place.

I got the impression that on a loaded system guests with a non-trivial
amount of vCPUs might be in trouble to be able to schedule them all
close enough for the rendezvous to not report a big skew, and thus
disable TSC_RELIABLE?

Thanks, Roger.