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    SubjectRe: [PATCH] clocksource, prevent overflow in clocksource_cyc2ns

    On 04/04/2012 09:08 PM, John Stultz wrote:
    > On 04/04/2012 11:33 AM, Prarit Bhargava wrote:
    >>> One idea might be to replace the cyc2ns w/ mult_frac in only the watchdog code.
    >>> I need to think on that some more (and maybe have you provide some debug output)
    >>> to really understand how that's solving the issue for you, but it would be able
    >>> to be done w/o affecting the other assumptions of the timekeeping core.
    >> Hey John,
    >> After reading the initial part of your reply I was thinking about calling
    >> mult_frac() directly from the watchdog code as well.
    >> Here's some debug output I cobbled together to get an idea of how quickly the
    >> overflow was happening.
    >> [ 5.435323] clocksource_watchdog: {0} cs tsc csfirst 227349443638728 mask
    >> 0xFFFFFFFFFFFFFFFF mult 797281036 shift 31
    >> [ 5.444930] clocksource_watchdog: {0} wd hpet wdfirst 78332535 mask
    >> 0xFFFFFFFF mult 292935555 shift 22
    >> These, of course, are just the basic data from the clocksources tsc and hpet.
    > If I'm doing the math right, these are ~2.7 Ghz cpus?


    > So what kernel version are you using?

    I was on an earlier version of Fedora (F16) ... but I'll jump forward and see if
    I can still hit it.

    > In trying to reproduce this locally against Linus' HEAD on a much smaller system
    > (single core + HT 1.6Ghz), I got:
    > [ 6.611366] clocksource_watchdog: {0} cs tsc csfirst 36177888648 mask
    > ffffffffffffffff mult 10485747 shift 24
    > [ 6.611596] clocksource_watchdog: {0} wd hpet wdfirst 169168400 mask ffffffff
    > mult 2684354560 shift 26
    > Note the smaller shift values. Not too long ago the shift calculation was
    > adjusted to allow for longer periods between interrupts, so I suspect you're on
    > an older kernel.
    > Further, using your debug patch on my system, it was well beyond 10 minutes
    > before the debug overflow occurred. And similarly I couldn't trip the watchdog
    > trigger using sysrq-t (but again, only two threads here, so not nearly as much
    > data to print as you have).

    I'm going to try this on a 32-cpu system (running the previously mentioned test)
    with linux.git HEAD.

    > Could you verify that the issue you're seeing is still is present w/ current
    > mainline? Please don't take this as me dismissing your problem! As I mentioned

    Absolutely :) I didn't take it that way at all. .... when I get in this AM I'll
    bang out a test and see if I can cause this to happen with sysrq-t. Keep in
    mind that 10000 threads is the *minimum* I was able to cause this with, which is
    only ~315 threads/cpu, which isn't a lot :/. At that number of threads the dump
    takes about 6 mins. Doubling it, IIRC, exceeded 10 mins.

    > earlier there are some known issues w/ the clocksource watchdog code. But I want
    > to narrow down if you're problem is currently present in mainline or only in
    > older kernels, as that will help us find the proper fix.

    Thanks John,


    > thanks
    > -john

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