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

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Date:	Tue, 2 Mar 2010 12:39:34 GMT
From:	tip-bot for Dimitri Sivanich <sivanich@....com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...hat.com,
	a.p.zijlstra@...llo.nl, venkatesh.pallipadi@...el.com,
	tglx@...utronix.de, sivanich@....com, mingo@...e.hu
Subject: [tip:sched/urgent] x86: Fix sched_clock_cpu for systems with unsynchronized TSC

Commit-ID:  14be1f7454ea96ee614467a49cf018a1a383b189
Gitweb:     http://git.kernel.org/tip/14be1f7454ea96ee614467a49cf018a1a383b189
Author:     Dimitri Sivanich <sivanich@....com>
AuthorDate: Mon, 1 Mar 2010 11:48:15 -0600
Committer:  Ingo Molnar <mingo@...e.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@....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>
---
 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;
 	}
 
 	/*
--
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