Re: [PATCH] sched, x86: fix overflow in cyc2ns_offset

[Paul Turner]

On Fri, Mar 9, 2012 at 4:00 PM, Salman Qazi <sqazi@xxxxxxxxxx> wrote:
> When a machine boots up, the TSC generally gets reset.  However, when
> kexec is used to boot into a kernel, the TSC value would be carried
> over from the previous kernel.  The computation of cycns_offset in
> set_cyc2ns_scale is prone to an overflow, if the machine has been up
> more than 208 days prior to the kexec.  The overflow happens when
> we multiply *scale, even though there is enough room to store the
> final answer.  We fix this issue by decomposing tsc_now into the
> quotient and remainder of division by CYC2NS_SCALE_FACTOR and then
> performing the multiplication separately on the two components.
>
> Refactor code to share the calculation with the previous
> fix in __cycles_2_ns.
>
> Signed-off-by: Salman Qazi <sqazi@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/timer.h |    8 ++------
>  arch/x86/kernel/tsc.c        |    3 ++-
>  include/linux/kernel.h       |   13 +++++++++++++
>  3 files changed, 17 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/include/asm/timer.h b/arch/x86/include/asm/timer.h
> index 431793e..34baa0e 100644
> --- a/arch/x86/include/asm/timer.h
> +++ b/arch/x86/include/asm/timer.h
> @@ -57,14 +57,10 @@ 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);
> -       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);
> +       ns += mult_frac(cyc, per_cpu(cyc2ns, cpu),
> +                       (1UL << CYC2NS_SCALE_FACTOR));
>        return ns;
>  }
>
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index a62c201..183c592 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -620,7 +620,8 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
>
>        if (cpu_khz) {
>                *scale = (NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR)/cpu_khz;
> -               *offset = ns_now - (tsc_now * *scale >> CYC2NS_SCALE_FACTOR);
> +               *offset = ns_now - mult_frac(tsc_now, *scale,
> +                                            (1UL << CYC2NS_SCALE_FACTOR));
>        }
>
>        sched_clock_idle_wakeup_event(0);
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index e834342..d801acb 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -85,6 +85,19 @@
>  }                                                      \
>  )
>
> +/*
> + * Multiplies an integer by a fraction, while avoiding unnecessary
> + * overflow or loss of precision.
> + */
> +#define mult_frac(x, numer, denom)(                    \
> +{                                                      \
> +       typeof(x) quot = (x) / (denom);                 \
> +       typeof(x) rem  = (x) % (denom);                 \
> +       (quot * (numer)) + ((rem * (numer)) / (denom)); \
> +}                                                      \
> +)

Any reason not to make this a static inline so that we get saner typing?

> +
> +
>  #define _RET_IP_               (unsigned long)__builtin_return_address(0)
>  #define _THIS_IP_  ({ __label__ __here; __here: (unsigned long)&&__here; })
>
>
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