Re: [PATCH RFC] x86/time: set AP's TSC scale estimate earlier

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, May 06, 2026 at 11:39:14AM +0200, Jan Beulich wrote:
> NOW() (in particular) can be used ahead of init_percpu_time(). As the
> initial scale value set is merely the BSP's, we can as well set it before
> actually launching the AP. Don't introduce yet another notifier function
> though; do this from smpboot.c's.
> 
> Setting the scale alone, however, doesn't work, so the entire struct
> cpu_time is copied.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
> RFC: Copying the entire struct won't work very well when tsc_adjust[] is
>      in use (and values there differ between sockets).
> 
> This in particular eliminates an anomaly with log messages issued early
> while APs are coming up, when "boot" console timestamps are in use.

Could we consider moving init_percpu_time() earlier in
start_secondary()?  I think it's main dependency is on
set_cpu_sibling_map(), which we could also move earlier?

Looking further, seems like it depends on smp_callin() having parsed
the CPUID features, plus the socket_cpumask[] also being set.

> --- a/xen/arch/x86/include/asm/time.h
> +++ b/xen/arch/x86/include/asm/time.h
> @@ -21,6 +21,7 @@ mktime (unsigned int year, unsigned int
>  int time_suspend(void);
>  int time_resume(void);
>  
> +void preinit_percpu_time(unsigned int cpu);
>  void init_percpu_time(void);
>  void time_latch_stamps(void);
>  
> --- a/xen/arch/x86/smpboot.c
> +++ b/xen/arch/x86/smpboot.c
> @@ -1139,6 +1139,7 @@ static int cf_check cpu_smpboot_callback
>      {
>      case CPU_UP_PREPARE:
>          rc = cpu_smpboot_alloc(cpu);
> +        preinit_percpu_time(cpu);
>          break;
>      case CPU_UP_CANCELED:
>      case CPU_DEAD:
> --- a/xen/arch/x86/time.c
> +++ b/xen/arch/x86/time.c
> @@ -2346,6 +2346,12 @@ void time_latch_stamps(void)
>      ap_bringup_ref.local_stime = get_s_time_fixed(ap_bringup_ref.local_tsc);
>  }
>  
> +void preinit_percpu_time(unsigned int cpu)
> +{
> +    /* Initial estimate for TSC rate etc. */
> +    per_cpu(cpu_time, cpu) = this_cpu(cpu_time);
> +}
> +
>  void init_percpu_time(void)
>  {
>      struct cpu_time *t = &this_cpu(cpu_time);
> @@ -2353,9 +2359,6 @@ void init_percpu_time(void)
>      u64 tsc;
>      s_time_t now;
>  
> -    /* Initial estimate for TSC rate. */
> -    t->tsc_scale = per_cpu(cpu_time, 0).tsc_scale;

Wouldn't it be simpler to pull this out of init_percpu_time() and do
it at the start of start_secondary()?

Thanks, Roger.