linux-kernel - Re: [PATCH 2/2] sched: Fix "divide error: 0000" in find_busiest

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Date:	Wed, 20 Jul 2011 00:33:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Terry Loftin <terry.loftin@...com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Bob Montgomery <bob.montgomery@...com>,
	John Stultz <johnstul@...ibm.com>
Subject: Re: [PATCH 2/2] sched: Fix "divide error: 0000" in
 find_busiest_group

On Tue, 2011-07-19 at 16:21 -0600, Terry Loftin wrote:
> > So you're running on a platform (unspecified) where we use a raw
> > sched_clock() that is buggy. Again, you're fixing symptoms not causes. 
> > 
> This x86_64.  This is the actual cause, unless the rq->clock
> value should never roll, in which case, the clock roll is the
> actual cause and you can disregard these patches.

Its supposed to roll over on the full 64bit, and I think x86_64 only
suffers this if you have sched_clock_stable set to 1.

So I think the correct fix is disabling that logic for now. John Stultz
was working on some patches to fix __cycles_2_ns().

Something like the below perhaps.

---
 arch/x86/kernel/cpu/intel.c |    2 --
 1 files changed, 0 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 1edf5ba..dba0482 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -91,8 +91,6 @@ 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);
-		if (!check_tsc_unstable())
-			sched_clock_stable = 1;
 	}
 
 	/*


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