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    SubjectRe: [RFC 1/2] time: workaround crappy hpet
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    On Mon, Apr 11, 2016 at 5:57 PM, Shaohua Li <shli@fb.com> wrote:
    > Calvin found 'perf record -a --call-graph dwarf -- sleep 5' making clocksource
    > switching to hpet. We found similar symptom in another machine. Here is an example:
    >
    > [8224517.520885] timekeeping watchdog: Marking clocksource 'tsc' as unstable, because the skew is too large:
    > [8224517.540032] 'hpet' wd_now: ffffffff wd_last: b39c0bd mask: ffffffff
    > [8224517.553092] 'tsc' cs_now: 48ceac7013714e cs_last: 48ceac25be34ac mask: ffffffffffffffff
    > [8224517.569849] Switched to clocksource hpet
    >
    > In both machines, wd_now is 0xffffffff. The tsc time looks correct, the cpu is 2.5G
    > (0x48ceac7013714e - 0x48ceac25be34ac)/2500000 = 0.4988s
    > 0.4988s matches WATCHDOG_INTERVAL. Since hpet reads to 0xffffffff in both
    > machines, this sounds not coincidence, hept is crappy.
    >
    > This patch tries to workaround this issue. We do retry if hpet has 0xffffff value.
    > In the relevant machine, the hpet counter doesn't read to 0xffffffff later.
    > The chance hpet has 0xffffffff counter is very small, this patch should have no
    > impact for good hpet.
    >
    > I'm open if there is better solution.

    Hrm..

    So can you characterize this bad behavior a bit more for us? Does
    every read of the HPET return 0xFFFFFFFF ? Or does it just
    occasionally start returning -1 values? Or once it trips and starts
    returning -1 does it always return -1?

    I'm trying to understand if there is a way to catch and disqualify
    that clocksource earlier then in the watchdog logic.

    thanks
    -john

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