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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

> --- 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)?

Jan

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