[tip:perf/core] x86/sched/perf/AMD: Set sched_clock_stable

From: tip-bot for Borislav Petkov
Date: Tue Feb 07 2012 - 14:44:04 EST

Commit-ID: c98fdeaa92731308ed80386261fa2589addefa47
Gitweb: http://git.kernel.org/tip/c98fdeaa92731308ed80386261fa2589addefa47
Author: Borislav Petkov <bp@xxxxxxxxx>
AuthorDate: Tue, 7 Feb 2012 13:08:52 +0100
Committer: Ingo Molnar <mingo@xxxxxxx>
CommitDate: Tue, 7 Feb 2012 13:12:08 +0100

x86/sched/perf/AMD: Set sched_clock_stable

Stephane Eranian reported that doing a scheduler latency
measurements with perf on AMD doesn't work out as expected due
to the fact that the sched_clock() granularity is too coarse,
i.e. done in jiffies due to the sched_clock_stable not set,
which, if set, would mean that we get to use the TSC as sample
source which would give us much higher precision.

However, there's no reason not to set sched_clock_stable on AMD
because all families from F10h and upwards do have an invariant
TSC and have the CPUID flag to prove (CPUID_8000_0007_EDX[8]).

Make it so, #1.

Signed-off-by: Borislav Petkov <bp@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Venki Pallipadi <venki@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Stephane Eranian <eranian@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxxxxx>
Cc: Robert Richter <robert.richter@xxxxxxx>
Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Cc: Andreas Herrmann <andreas.herrmann3@xxxxxxx>
Link: 20120206132546.GA30854@quad">http://lkml.kernel.org/r/20120206132546.GA30854@quad
[ Should any non-standard system break the TSC, we should
mark them so explicitly, in their platform init handler, or
in a DMI quirk. ]
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
arch/x86/kernel/cpu/amd.c | 3 +++
1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index f4773f4..0a44b90 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -5,6 +5,7 @@
#include <linux/mm.h>

#include <linux/io.h>
+#include <linux/sched.h>
#include <asm/processor.h>
#include <asm/apic.h>
#include <asm/cpu.h>
@@ -456,6 +457,8 @@ static void __cpuinit early_init_amd(struct cpuinfo_x86 *c)
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;

#ifdef CONFIG_X86_64
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