Messages in this thread | | | Subject | Re: [PATCH] sched, x86: fix overflow in cyc2ns_offset | From | john stultz <> | Date | Fri, 09 Mar 2012 16:25:40 -0800 |
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On Fri, 2012-03-09 at 16:00 -0800, Salman Qazi 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.
Thanks so much for making it more generic and reusable! But one question below.
> Signed-off-by: Salman Qazi <sqazi@google.com> > --- > 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)); \ > +} \ > +) > + > +
So... Sorry, why did you change it from the shifted logic?
I was thinking more like: mult_shifted_fract(x, mult, shift) { quot = x >> shift; rem = x & ((1ULL << shift)-1); return quot * mult + (rem * mult) >> shift; }
Is the compiler really smart enough to avoid the divides?
thanks -john
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