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

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Message-ID: <20100301174815.GC8224@sgi.com>
Date:	Mon, 1 Mar 2010 11:48:15 -0600
From:	Dimitri Sivanich <sivanich@....com>
To:	linux-kernel@...r.kernel.org
Cc:	"H. Peter Anvin" <hpa@...or.com>, venkatesh.pallipadi@...el.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: [PATCH v2] 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>

---

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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