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

From: Borislav Petkov
Date: Mon Feb 06 2012 - 10:34:45 EST

+ 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.



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