Messages in this thread |  | | | Date | Fri, 9 Mar 2012 16:29:15 -0800 | | Subject | Re: [PATCH] sched, x86: fix overflow in cyc2ns_offset | | From | Salman Qazi <> |
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On Fri, Mar 9, 2012 at 4:22 PM, Paul Turner <pjt@google.com> wrote: > On Fri, Mar 9, 2012 at 4:00 PM, Salman Qazi <sqazi@google.com> 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@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)); \ >> +} \ >> +) > > Any reason not to make this a static inline so that we get saner typing?
Only re-usability. I put it in include/linux/kernel.h with a lot of other #defines like min , max, clamp, that are generally applicable.
> >> + >> + >> #define _RET_IP_ (unsigned long)__builtin_return_address(0) >> #define _THIS_IP_ ({ __label__ __here; __here: (unsigned long)&&__here; }) >> >> -- 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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