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Message-ID: <1331320982.26253.114.camel@work-vm>
Date: Fri, 09 Mar 2012 11:23:02 -0800
From: john stultz <johnstul@...ibm.com>
To: Salman Qazi <sqazi@...gle.com>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>,
LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH] sched, x86: fix overflow in cyc2ns_offset
On Thu, 2012-03-08 at 15:23 -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.
>
> Signed-off-by: Salman Qazi <sqazi@...gle.com>
> ---
> arch/x86/kernel/tsc.c | 12 +++++++++++-
> 1 files changed, 11 insertions(+), 1 deletions(-)
>
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index a62c201..ef1dc8e 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -608,6 +608,8 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
> {
> unsigned long long tsc_now, ns_now, *offset;
> unsigned long flags, *scale;
> + unsigned long long quot;
> + unsigned long long rem;
>
> local_irq_save(flags);
> sched_clock_idle_sleep_event();
> @@ -620,7 +622,15 @@ 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);
> +
> + /*
> + * Avoid premature overflow by splitting into quotient
> + * and remainder. See the comment above __cycles_2_ns
> + */
> + quot = (tsc_now >> CYC2NS_SCALE_FACTOR);
> + rem = tsc_now & ((1ULL << CYC2NS_SCALE_FACTOR) - 1);
> + *offset = ns_now - (quot * *scale +
> + ((rem * *scale) >> CYC2NS_SCALE_FACTOR));
> }
This clearly is a needed fix. Thanks for finding it and sending it in.
Although I'm curious if it might be good to encapsulate this code into a
macro that can be reused in both set_cyc2ns_scale and __cycles_2_ns()
(as well as others, I suspect this issue is going to crop up on other
arches at some point too)?
thanks
-john
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