Re: [BUG] perf: perf sched warning possibly due to clock granularity on AMD

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2012-02-06 at 16:34 +0100, Borislav Petkov wrote:
> + Andreas.
> 
> On Mon, Feb 06, 2012 at 03:26:43PM +0100, Peter Zijlstra wrote:
> > On Mon, 2012-02-06 at 14:25 +0100, Stephane Eranian wrote:
> > > In summary, two issues:
> > >   - Why is sched_clock_stable not set or even tested on recent AMD systems?
> 
> AFAICT, sched_clock_stable is set on Intel under the following conditions:
> 
>         /*
>          * c->x86_power is 8000_0007 edx. Bit 8 is TSC runs at constant rate
>          * with P/T states and does not stop in deep C-states.
>          *
>          * It is also reliable across cores and sockets. (but not across
>          * cabinets - we turn it off in that case explicitly.)
>          */
>         if (c->x86_power & (1 << 8)) {
>                 set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
>                 set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
>                 if (!check_tsc_unstable())
>                         sched_clock_stable = 1;
>         }
> 
> and yes, we can do CONSTANT_TSC and NONSTOP_TSC on the now older F10h
> already, so Bulldozer can do that too, implicitly.
> 
> I don't see why sched_clock_stable shouldn't be moved to generic x86
> code and set based on the two CPUID cap flags above.

stable means more than both those flags together.. 

 - CONSTANT_TSC means freq independent
 - NONSTOP_TSC means it doesn't get stopped in any C state

Together they make TSC completely C-state independent.
sched_clock_stable actually means that TSC is both _AND_ is consistent
across all CPUs in the entire system.