Re: [Xen-devel] [PATCH] x86/time: use correct (local) time stamp in constant-TSC calibration fast path

[Joao Martins <joao.m.martins@xxxxxxxxxx>]

On 06/09/2016 01:01 PM, Jan Beulich wrote:
> This looks like a copy and paste mistake in commit 1b6a99892d ("x86:
> Simpler time handling when TSC is constant across all power saving
> states"), responsible for occasional many-microsecond cross-CPU skew of
> what NOW() returns.
> 
> Also improve the correlation between local TSC and stime stamps
> obtained at the end of the two calibration handlers: Compute the stime
> one from the TSC one, instead of doing another rdtsc() for that
> compuation.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> As to 4.7 inclusion: This of course looks like a pretty blatant mistake
> that has been there for many years (and hence many releases). There's
> certainly non-zero risk that I'm overlooking something here (despite
> Joao apparently having come to the same conclusion), so I can't really
> make up my mind on whether to request this patch to be put there right
> away, or rather having linger in -unstable for a while.
> 
Initially I thought of this as a fix too, but then wouldn't having
t->stime_local_stamp be c->stime_local_stamp, render no use to the
platform timer reads done on calibration? Unless we would change
update_vcpu_system to use stime_master_stamp instead?
stime_master_stamp field isn't used anywhere other than the dom0 injected
cpu_frequency_change or when at boot seeding the cpu_time struct on
init_percpu_time (and the already mentioned use on local_time_calibration) ?
init_percpu_time also takes a different read of the platform timer per
cpu and could probably be inherited by a read done on the boot processor
and written on remaining CPUs, so that all would start from the same stamp.
IOW - this sounds like time we are turning stime to be totally TSC except
when initially seeding each cpu_time?

> --- a/xen/arch/x86/time.c
> +++ b/xen/arch/x86/time.c
> @@ -998,7 +998,7 @@ static void local_time_calibration(void)
>          /* Atomically read cpu_calibration struct and write cpu_time struct. 
> */
>          local_irq_disable();
>          t->local_tsc_stamp    = c->local_tsc_stamp;
> -        t->stime_local_stamp  = c->stime_master_stamp;
> +        t->stime_local_stamp  = c->stime_local_stamp;
>          t->stime_master_stamp = c->stime_master_stamp;
>          local_irq_enable();
>          update_vcpu_system_time(current);
> @@ -1275,7 +1275,7 @@ static void time_calibration_tsc_rendezv
>      }
>  
>      c->local_tsc_stamp = rdtsc();
> -    c->stime_local_stamp = get_s_time();
> +    c->stime_local_stamp = get_s_time_fixed(c->local_tsc_stamp);
>      c->stime_master_stamp = r->master_stime;
>  
>      raise_softirq(TIME_CALIBRATE_SOFTIRQ);
> @@ -1305,7 +1305,7 @@ static void time_calibration_std_rendezv
>      }
>  
>      c->local_tsc_stamp = rdtsc();
> -    c->stime_local_stamp = get_s_time();
> +    c->stime_local_stamp = get_s_time_fixed(c->local_tsc_stamp);
>      c->stime_master_stamp = r->master_stime;
>  
>      raise_softirq(TIME_CALIBRATE_SOFTIRQ);
> 
> 
> 

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