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    SubjectRe: [BISECTED] 2.6.35.*: horrible (exponential? >linear) slowdown to unusability (HPET)
    On Tue, 14 Sep 2010, Nix wrote:

    > On 14 Sep 2010, Thomas Gleixner said:
    >
    > > On Fri, 10 Sep 2010, Thomas Gleixner wrote:
    > >> On Fri, 10 Sep 2010, Nix wrote:
    > >> >
    > >> > I suspect current_clocksource doesn't do what we think it does, or the
    > >> > clocksource and 'clock event device' are not the same.)
    > >>
    > >> Right, they are not the same. clocksource provides us a read out
    > >> device for timekeeping (usually a simple increasing counter). clock
    > >> event device is used to generate timer interrupts.
    > >>
    > >> HPET provides both functionalities.
    > >>
    > >> The patch you bisected is affecting the clock events part of the
    > >> HPET. So yes, it's not a clock source problem.
    > >
    > > Another question. If you add hpet=verbose (or revert that commit) does
    > > the printk_once in hpet_set_next_event() show up in dmesg ?
    > >
    > > "hpet: compare register read back failed."
    >
    > Well, given that I reverted that commit in the past (during the
    > bisection) and also added hpet=verbose[1], I'd have expected it to turn
    > up in my past syslogs. There is no sign of it, in syslogs back to 2009.
    >
    >
    > [1] and indeed am running 2.6.35.4 with hpet=verbose and no symptoms
    > on the affected system *now* -- a mystery because I thought the
    > hpet=verbose-turns-it-off code was removed, but turning on verbose
    > still seems to quash the bug?! or perhaps it's slightly
    > intermittent and every N boots I'm lucky enough to miss it. I'm
    > slightly chary of rebooting to find out ;}

    hpet=verbose forces the read back.

    FYI, we got a reasonable explanation now what's going on in hardware
    land. Patch is in the works.

    Thanks,

    tglx



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