Re: [Xen-devel] [PATCH] x86/tsc: limit the usage of synchronization rendezvous

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

On Wed, Oct 23, 2019 at 04:17:11PM +0200, Jan Beulich wrote:
> On 23.10.2019 15:56, Roger Pau Monne wrote:
> > If Xen detects the TSC is unreliable it will set a rendezvous helper
> > that tries to synchronize the different CPUs TSC value by propagating
> > the one from CPU#0 and writing it into the IA32_TSC MSR on each CPU.
> > 
> > When the system has a single thread and there are no hotplugable CPUs
> > doing the above just results in reading the TSC from CPU#0 and writing
> > it into the IA32_TSC MSR of CPU#0, which is pointless, so limit the
> > usage of the synchronization rendezvous to systems that have more than
> > one CPU, even if those CPUs are yet to be hotplugged.
> > 
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > ---
> > I'm not sure whether this is suitable for 4.13, being a performance
> > improvement but not fixing a functional bug.
> 
> Unless we know there's a lot of use of Xen in UP mode, I'd say
> rather not. _If_ there was a lot of such use, then I think we
> ought to do more work towards performance there (like re-
> introducing SMP alternatives patching).
> 
> Of course in the end the decision is to be taken by Jürgen,
> whom you didn't even Cc.

Since this is a performance fix I felt like it wasn't really
appropriate for 4.13, hence I didn't Cc Jürgen but added this note.

> > --- a/xen/arch/x86/time.c
> > +++ b/xen/arch/x86/time.c
> > @@ -1821,7 +1821,8 @@ static int __init verify_tsc_reliability(void)
> >       * are not marked as 'reliable', re-sync during rendezvous.
> >       */
> >      if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
> > -         !boot_cpu_has(X86_FEATURE_TSC_RELIABLE) )
> > +         !boot_cpu_has(X86_FEATURE_TSC_RELIABLE) &&
> > +         num_present_cpus() > 1 )
> >          time_calibration_rendezvous_fn = time_calibration_tsc_rendezvous;
> 
> Did you consider the alternative of switching the rendezvous
> function when going from UP to MP mode (or back)?

I will look into that. The usefulness of this fix is quite limited,
given it only applies to UP, so I didn't want to add a lot of
complexity for a not very common scenario IMO.

Thanks, Roger.

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