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    SubjectRe: [PATCH] hangcheck-timer is broken on x86
    >> > What I'm saying is that if you're using getrawmonotonic() to detect
    >> > hangs, you might miss them, as getrawmonotonic may wrap (and thus stop
    >> > continually increasing) if the timer interrupt is delayed. This does not
    >> > apply to systems using the TSC clocksource, but does apply to systems
    >> > using the acpi_pm.
    >> But if timer interrupt is delayed by more than acpi_pm wrap-around
    >> time, then the update_wall_time() is also screwed. Since it is not, we
    >> can rely on getrawmonotonic().
    > Right, if the box hangs for longer then the clocksource can count for,
    > the timekeeping subsystem will be off by some multiple of that length.

    Oh, I see. You mean that getrawmonotonic() wouldn't work under
    abnormal conditions. I understand now, sorry for the confusion. You
    are correct, of course.

    I personally don't like the idea of relying on read_persistent_clock()
    not only because of hwclock and ntp. In fact, my core interest in
    hangcheck-timer is to set a very low margin (1 to 3 jiffies for
    example) so that I would get a log message upon any kernel slow down
    or a tick-miss (as a hardware integrity check). I don't think
    read_persistent_clock() is precise enough for this purpose, is it?

    Also, hooking to ntp update code complicates an otherwise simple
    driver. I propose to simply check on non-S390 if the clock source
    resolves to something other than TSC and dump a warning message on
    driver load (something like "Hangcheck: kernel using clocksource %s,
    which is not reliable for hang detection").

    What do you think about it?


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