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    SubjectRe: [PATCH 1/2] nohz: Disable LOCKUP_DETECTOR when NO_HZ_FULL is enabled
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    On Wed, 2013-05-15 at 11:06 -0400, Don Zickus wrote:

    > > That's a bit contradictory in function, you want the NMI watchdog to
    > > cover all code, so disabling whilst entering NO_HZ state is going to
    > > make it not cover some code - *fail*.

    Well, when NO_HZ_FULL is set, it covers no code :-)

    > >
    > > Rather I would suggest disabling the NMI watchdog's runtime default; so
    > > you can still enable it with something like:
    > >
    > > echo 1 > /proc/sys/kernel/nmi_watchdog

    Yeah, just disabling it via run time might work.


    >
    > Coming into the middle of the thread is always hard, but why/how does perf
    > disable nohz_full? I didn't think the hardware events of perf would cause
    > problems as they are no different than an irq. Curious.

    Right now perf requires a tick, not sure exactly why, but you can look
    at the code in perf_event_task_tick(). Thus if NO_HZ_FULL sees that a
    perf tick is pending, it won't disable ticks. Unfortunately, the
    watchdogs, both NMI and soft lockup, use the perf infrastructure to
    trigger NMIs or interrupts. This adds a perf element on the rotate list
    and keeps NO_HZ_FULL from *ever* activating.

    -- Steve




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