Messages in this thread | | | Date | Fri, 9 Mar 2012 16:28:05 -0800 | Subject | Re: [PATCH] sched, x86: fix overflow in cyc2ns_offset | From | Salman Qazi <> |
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On Fri, Mar 9, 2012 at 4:25 PM, john stultz <johnstul@us.ibm.com> wrote: > > 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? >
It's more generally applicable. The compiler is smart enough to do the right thing in the power-of-two constant case.
> 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?
Yes, it is. I verified that.
> > thanks > -john > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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