linux-kernel - [115/116] x86: Fix sched_clock_cpu for systems with unsynchronized TSC

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Message-Id: <20100330225715.375996152@linux.site>
Date:	Tue, 30 Mar 2010 15:56:33 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Dimitri Sivanich <sivanich@....com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: [115/116] x86: Fix sched_clock_cpu for systems with unsynchronized TSC

2.6.32-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Dimitri Sivanich <sivanich@....com>

commit 14be1f7454ea96ee614467a49cf018a1a383b189 upstream.

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@....com>
Acked-by: Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
LKML-Reference: <20100301174815.GC8224@....com>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

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

--- a/arch/x86/kernel/cpu/intel.c
+++ b/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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