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    SubjectRe: [watchdog] combine nmi_watchdog and softlockup
    On Fri, Apr 09, 2010 at 02:00:38AM +0200, Frederic Weisbecker wrote:
    > On Tue, Apr 06, 2010 at 07:31:15PM +0400, Cyrill Gorcunov wrote:
    > > > I fear the cpu clock is not going to help you detecting any hard lockups.
    > > > If you're stuck in an interrupt or an irq disabled loop, your cpu clock is
    > > > not going to fire.
    > > >
    > >
    > > I guess it's not supposed to. For such cases only nmi irqs may help for which
    > > the perf events are there (/me need to check if we program apic timer for anything
    > > like that). But it should help for other deadlocks. Or I miss something?
    >
    >
    > Actually not. What the hardlockup detector does it to check the progression
    > of irqs.
    >

    yup, i know what nmi-watchdog is doing. I guess you've misunderstood me. I meant
    that sw-driven detector is not supposed to guard against the cases you're
    referring to. I don't remember the details but someone proposed to make a
    fallback to sw-watchdog if there is no ability to use nmi from perf-events
    (for any reason) which eventually being implemented in Don's patch. And
    there will be a message that watchdog has been switched to sw-driven
    scaffold. So user will (or should) see this message and mark it I believe.
    This sw-watchdog is like "ok, we've been trying our best but there is a
    problem and the only solution we could offer -- is to use sw-watchdog".
    That is how I understand the reason for sw-watchdog there.

    >
    > So it detects true hardlockups: stuck in an irq disabled section.
    > If you don't have NMI to detect that (here this made by hardware clock based
    > on cpu cycles overflows), then you're screwed. The hardlockup detector is
    > useless with a maskable irq based clock.
    >
    -- Cyrill


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