Re: [Xen-devel] [PATCH 1/8] x86/time: improve cross-CPU clock monotonicity (and more)

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.06.16 at 12:26, <JBeulich@xxxxxxxx> wrote:

The title of this was (of course) meant to be

x86/time: adjust local system time initialization

Jan

> Using the bare return value from read_platform_stime() is not suitable
> when local_time_calibration() is going to use its fast path: Divergence
> of several dozen microseconds between NOW() return values on different
> CPUs results when platform and local time don't stay in close sync.
> 
> Latch local and platform time on the CPU initiating AP bringup, such
> that the AP can use these values to seed its stime_local_stamp with as
> little of an error as possible. The boot CPU, otoh, can simply
> calculate the correct initial value (other CPUs could do so too with
> even greater accuracy than the approach being introduced, but that can
> work only if all CPUs' TSCs start ticking at the same time, which
> generally can't be assumed to be the case on multi-socket systems).
> 
> This slightly defers init_percpu_time() (moved ahead by commit
> dd2658f966 ["x86/time: initialise time earlier during
> start_secondary()"]) in order to reduce as much as possible the gap
> between populating the stamps and consuming them.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> --- a/xen/arch/x86/smpboot.c
> +++ b/xen/arch/x86/smpboot.c
> @@ -328,12 +328,12 @@ void start_secondary(void *unused)
>  
>      percpu_traps_init();
>  
> -    init_percpu_time();
> -
>      cpu_init();
>  
>      smp_callin();
>  
> +    init_percpu_time();
> +
>      setup_secondary_APIC_clock();
>  
>      /*
> @@ -996,6 +996,8 @@ int __cpu_up(unsigned int cpu)
>      if ( (ret = do_boot_cpu(apicid, cpu)) != 0 )
>          return ret;
>  
> +    time_latch_stamps();
> +
>      set_cpu_state(CPU_STATE_ONLINE);
>      while ( !cpu_online(cpu) )
>      {
> --- a/xen/arch/x86/time.c
> +++ b/xen/arch/x86/time.c
> @@ -1328,21 +1328,51 @@ static void time_calibration(void *unuse
>                       &r, 1);
>  }
>  
> +static struct {
> +    s_time_t local_stime, master_stime;
> +} ap_bringup_ref;
> +
> +void time_latch_stamps(void) {
> +    unsigned long flags;
> +    u64 tsc;
> +
> +    local_irq_save(flags);
> +    ap_bringup_ref.master_stime = read_platform_stime();
> +    tsc = rdtsc();
> +    local_irq_restore(flags);
> +
> +    ap_bringup_ref.local_stime = get_s_time_fixed(tsc);
> +}
> +
>  void init_percpu_time(void)
>  {
>      struct cpu_time *t = &this_cpu(cpu_time);
>      unsigned long flags;
> +    u64 tsc;
>      s_time_t now;
>  
>      /* Initial estimate for TSC rate. */
>      t->tsc_scale = per_cpu(cpu_time, 0).tsc_scale;
>  
>      local_irq_save(flags);
> -    t->local_tsc_stamp = rdtsc();
>      now = read_platform_stime();
> +    tsc = rdtsc();
>      local_irq_restore(flags);
>  
>      t->stime_master_stamp = now;
> +    /*
> +     * To avoid a discontinuity (TSC and platform clock can't be expected
> +     * to be in perfect sync), initialization here needs to match up with
> +     * local_time_calibration()'s decision whether to use its fast path.
> +     */
> +    if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) )
> +    {
> +        if ( system_state < SYS_STATE_smp_boot )
> +            now = get_s_time_fixed(tsc);
> +        else
> +            now += ap_bringup_ref.local_stime - ap_bringup_ref.master_stime;
> +    }
> +    t->local_tsc_stamp    = tsc;
>      t->stime_local_stamp  = now;
>  }
>  
> --- a/xen/include/asm-x86/time.h
> +++ b/xen/include/asm-x86/time.h
> @@ -40,6 +40,7 @@ int time_suspend(void);
>  int time_resume(void);
>  
>  void init_percpu_time(void);
> +void time_latch_stamps(void);
>  
>  struct ioreq;
>  int hwdom_pit_access(struct ioreq *ioreq);




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