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

[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 <sivanich@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;
 	}
 
 	/*

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

[Pallipadi, Venkatesh]

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

On Mon, 2010-03-01 at 09:48 -0800, Dimitri Sivanich wrote:
> 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 <sivanich@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;
>  	}
>  
>  	/*

[tip:sched/urgent] x86: Fix sched_clock_cpu for systems with unsynchronized TSC

[tip-bot for Dimitri Sivanich]

Commit-ID:  14be1f7454ea96ee614467a49cf018a1a383b189
Gitweb:     http://git.kernel.org/tip/14be1f7454ea96ee614467a49cf018a1a383b189
Author:     Dimitri Sivanich <sivanich@sgi.com>
AuthorDate: Mon, 1 Mar 2010 11:48:15 -0600
Committer:  Ingo Molnar <mingo@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 <sivanich@sgi.com>
Acked-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100301174815.GC8224@sgi.com>
Signed-off-by: Ingo Molnar <mingo@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)
 	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;
 	}
 
 	/*