Ack: SRU: sched, x86: Avoid unnecessary overflow in sched_clock
 

On Wed, Jan 11, 2012 at 03:13:02PM -0600, Chris J Arges wrote:
> SRU Justification:
>
> BugLink: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/805341
>
> Impact: Machines with uptimes of 200+ days will sometimes have soft lockups.
>
> Fix: This was fixed in upstream. It has already been backported to Lucid.
>
> Testcase: Run your computer for 200+ days and verify it doesn't lockup.

> From a9747c4dc863ab62553dd65c4159b4b8e749042f Mon Sep 17 00:00:00 2001
> From: Salman Qazi <sqazi@google.com>
> Date: Wed, 11 Jan 2012 12:29:48 -0600
> Subject: [PATCH] sched, x86: Avoid unnecessary overflow in sched_clock
>
> (Added the missing signed-off-by line)
>
> In hundreds of days, the __cycles_2_ns calculation in sched_clock
> has an overflow. cyc * per_cpu(cyc2ns, cpu) exceeds 64 bits, causing
> the final value to become zero. We can solve this without losing
> any precision.
>
> We can decompose TSC into quotient and remainder of division by the
> scale factor, and then use this to convert TSC into nanoseconds.
>
> Signed-off-by: Salman Qazi <sqazi@google.com>
> Acked-by: John Stultz <johnstul@us.ibm.com>
> Reviewed-by: Paul Turner <pjt@google.com>
> Cc: stable@kernel.org
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Link: http://lkml.kernel.org/r/20111115221121.7262.88871.stgit@dungbeetle.mtv.cor p.google.com
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
> (cherry picked from commit 4cecf6d401a01d054afc1e5f605bcbfe553cb9b9)
>
> Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
> BugLink: http://bugs.launchpad.net/bugs/805341
> ---
> arch/x86/include/asm/timer.h | 23 ++++++++++++++++++++++-
> 1 files changed, 22 insertions(+), 1 deletions(-)
>
> diff --git a/arch/x86/include/asm/timer.h b/arch/x86/include/asm/timer.h
> index fa7b917..431793e 100644
> --- a/arch/x86/include/asm/timer.h
> +++ b/arch/x86/include/asm/timer.h
> @@ -32,6 +32,22 @@ extern int no_timer_check;
> * (mathieu.desnoyers@polymtl.ca)
> *
> * -johnstul@us.ibm.com "math is hard, lets go shopping!"
> + *
> + * In:
> + *
> + * ns = cycles * cyc2ns_scale / SC
> + *
> + * Although we may still have enough bits to store the value of ns,
> + * in some cases, we may not have enough bits to store cycles * cyc2ns_scale,
> + * leading to an incorrect result.
> + *
> + * To avoid this, we can decompose 'cycles' into quotient and remainder
> + * of division by SC. Then,
> + *
> + * ns = (quot * SC + rem) * cyc2ns_scale / SC
> + * = quot * cyc2ns_scale + (rem * cyc2ns_scale) / SC
> + *
> + * - sqazi@google.com
> */
>
> DECLARE_PER_CPU(unsigned long, cyc2ns);
> @@ -41,9 +57,14 @@ DECLARE_PER_CPU(unsigned long long, cyc2ns_offset);
>
> static inline unsigned long long __cycles_2_ns(unsigned long long cyc)
> {
> + unsigned long long quot;
> + unsigned long long rem;
> int cpu = smp_processor_id();
> unsigned long long ns = per_cpu(cyc2ns_offset, cpu);
> - ns += cyc * per_cpu(cyc2ns, cpu) >> CYC2NS_SCALE_FACTOR;
> + quot = (cyc >> CYC2NS_SCALE_FACTOR);
> + rem = cyc & ((1ULL << CYC2NS_SCALE_FACTOR) - 1);
> + ns += quot * per_cpu(cyc2ns, cpu) +
> + ((rem * per_cpu(cyc2ns, cpu)) >> CYC2NS_SCALE_FACTOR);
> return ns;
> }

Clean cherry-pick. The math looks correct.

Acked-by: Seth Forshee <seth.forshee@canonical.com>

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