Re: x86 TSC time warp puzzle

[Joe Korty <joe.korty@ccur.com>]

On Mon, Apr 04, 2005 at 09:59:22AM +0100, P@draigBrady.com wrote:
> Jonathan Lundell wrote:
> >Well, not actually a time warp, though it feels like one.
> >
> >I'm doing some real-time bit-twiddling in a driver, using the TSC to 
> >measure out delays on the order of hundreds of nanoseconds. Because I 
> >want an upper limit on the delay, I disable interrupts around it.
> >
> >The logic is something like:
> >
> >    local_irq_save
> >    out(set a bit)
> >    t0 = TSC
> >    wait while (t = (TSC - t0)) < delay_time
> >    out(clear the bit)
> >    local_irq_restore
> >
> > From time to time, when I exit the delay, t is *much* bigger than 
> >delay_time. If delay_time is, say, 300ns, t is usually no more than 
> >325ns. But every so often, t can be 2000, or 10000, or even much higher.
> >
> >The value of t seems to depend on the CPU involved, The worst case is 
> >with an Intel 915GV chipset, where t approaches 500 microseconds (!).


Add nmi_watchdog=0 to your boot command line.