Messages in this thread |  | | Date | Tue, 21 Oct 2014 09:11:04 -0700 | Subject | Re: [PATCH 1/2] time: Fix NTP adjustment mult overflow. | From | John Stultz <> |
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On Tue, Oct 21, 2014 at 3:19 AM, Thomas Gleixner <tglx@linutronix.de> wrote: > On Thu, 9 Oct 2014, pang.xunlei wrote: > > First of all: Please use proper line breaks in the changelog. > >> The mult memember of struct clocksource should always be a large u32 >> number when calculated through __clocksource_updatefreq_scale(). The >> value of (cs->mult+cs->maxadj) may have a chance to reach very near >> 0xFFFFFFFF. > > And what's the actual problem with reaching a value near 0xFFFFFFFF? > >> For instance, 555MHz oscillator: cs->mult is 0xE6A17102, >> cs->maxadj is 0x195E8EFD, cs->mult+cs->maxadj is 0xFFFFFFFF. Such >> oscillators would probably exist on some processors like MIPS which >> use CP0 compare/count CPU clock as the clock source. > > Again, what's the problem?
So the problem is that with an adjustment the mult value might overflow, becoming very small, bascially causing time to stop increasing. This is mentioned below, but I agree we're burying the headline a bit.
>> Clocksource might encounter large frequency adjustment due to the >> hardware unstability, environment temperature, software deviation, >> NTP algorithm accuracy, etc. When NTP slewes the clock, kernel goes >> through update_wall_time()->...->timekeeping_apply_adjustment(): >> tk->tkr.mult += mult_adj; Unfortunately, tk->tkr.mult may overflow >> after this operation, though such cases are next to impossible to >> happen in practice. > > So you adding this just for correctness reasons, not because you > observed the problem in practice?
This is my understanding.
I'll work with Xunlei to make further clarifications to the changelog to make this more explicit.
Thanks for your feedback! -john
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