[PATCH v2] x86: Fix sched_clock_cpu for systems with unsynchronized TSC

[Dimitri Sivanich]

On UV systems, the TSC is not synchronized across blades. The
sched_clock_cpu() function is returning values that can go backwards
(I've seen as much as 8 seconds) when switching between cpus.

As each cpu comes up, early_init_intel() will currently set the
sched_clock_stable flag true. When mark_tsc_unstable() runs, it clears
the flag, but this only occurs once (the first time a cpu comes up whose
TSC is not synchronized with cpu 0). After this, early_init_intel() will
set the flag again as the next cpu comes up.

Only set sched_clock_stable if tsc has not been marked unstable.

Signed-off-by: Dimitri Sivanich <siva...@sgi.com>

---

Only affects x86 arch.

arch/x86/kernel/cpu/intel.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

Index: linux/arch/x86/kernel/cpu/intel.c
===================================================================
--- linux.orig/arch/x86/kernel/cpu/intel.c
+++ linux/arch/x86/kernel/cpu/intel.c
@@ -70,7 +70,8 @@ static void __cpuinit early_init_intel(s
if (c->x86_power & (1 << 8)) {
set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
- sched_clock_stable = 1;
+ if (!check_tsc_unstable())
+ sched_clock_stable = 1;
}

/*
--
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[Pallipadi, Venkatesh]

Acked-by: Venkatesh Pallipadi <venkatesh...@intel.com>

[tip-bot for Dimitri Sivanich]

Commit-ID: 14be1f7454ea96ee614467a49cf018a1a383b189
Gitweb: http://git.kernel.org/tip/14be1f7454ea96ee614467a49cf018a1a383b189
Author: Dimitri Sivanich <siva...@sgi.com>
AuthorDate: Mon, 1 Mar 2010 11:48:15 -0600
Committer: Ingo Molnar <mi...@elte.hu>
CommitDate: Tue, 2 Mar 2010 13:36:11 +0100

x86: Fix sched_clock_cpu for systems with unsynchronized TSC

On UV systems, the TSC is not synchronized across blades. The
sched_clock_cpu() function is returning values that can go
backwards (I've seen as much as 8 seconds) when switching
between cpus.

As each cpu comes up, early_init_intel() will currently set the
sched_clock_stable flag true. When mark_tsc_unstable() runs, it
clears the flag, but this only occurs once (the first time a cpu
comes up whose TSC is not synchronized with cpu 0). After this,
early_init_intel() will set the flag again as the next cpu comes
up.

Only set sched_clock_stable if tsc has not been marked unstable.

Signed-off-by: Dimitri Sivanich <siva...@sgi.com>
Acked-by: Venkatesh Pallipadi <venkatesh...@intel.com>
Acked-by: Peter Zijlstra <a.p.zi...@chello.nl>
LKML-Reference: <2010030117...@sgi.com>
Signed-off-by: Ingo Molnar <mi...@elte.hu>
---
arch/x86/kernel/cpu/intel.c | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 879666f..7e1cca1 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -70,7 +70,8 @@ static void __cpuinit early_init_intel(struct cpuinfo_x86 *c)