Re: [Xen-devel] [PATCH] Adjust time init sequence

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

Is it really safe to use NOW() before init_percpu_time()? Seems dodgy.

Since calibration points are sync'ed across all CPUs on Xen 3.3 and
unstable, trying to ensure that the first calibration period across all CPUs
is equal is a lost cause isn't it?

 -- Keir

On 10/12/2008 13:10, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:

> Adjust time init sequence
> 
> percpu timer init for BSP happens within init_xen_time,
> while CMOS access at end may take up to 1s. This may
> make 1st trigger to calibration timer happens >1s interval
> and elapsed local stime for BSP also exceed 1s. However
> percpu timer init happens after that point for APs, which
> will then have a <1s elapsed local stime at 1st calibration.
> That leads to distinct mul_frac among cores, which can
> cause up to 6ms system time skew in the start. This is
> not big issue, since gradually xen calibration framework
> closes that gap. But it's still better to avoid that big
> skew in early boot phase.
> 
> Signed-off-by Kevin Tian <kevin.tian@xxxxxxxxx>
> 
> diff -r 85b2f4aafea4 xen/arch/x86/time.c
> --- a/xen/arch/x86/time.c Tue Dec 09 20:56:23 2008 -0500
> +++ b/xen/arch/x86/time.c Tue Dec 09 22:21:07 2008 -0500
> @@ -1095,7 +1095,7 @@
>  
>      local_irq_save(flags);
>      rdtscll(t->local_tsc_stamp);
> -    now = !plt_src.read_counter ? 0 : read_platform_stime();
> +    now = read_platform_stime();
>      local_irq_restore(flags);
>  
>      t->stime_master_stamp = now;
> @@ -1118,12 +1118,13 @@
>  
>      open_softirq(TIME_CALIBRATE_SOFTIRQ, local_time_calibration);
>  
> -    init_percpu_time();
> +    do_settime(get_cmos_time(), 0, 0);
>  
>      stime_platform_stamp = 0;
>      init_platform_timer();
>  
> -    do_settime(get_cmos_time(), 0, NOW());
> +    /* init bsp percpu time after platform timer initialized, similar as AP
> */
> +    init_percpu_time();
>  
>      return 0;
>  }
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