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    SubjectNMI watchdog setup_lapic_nmi_watchdog() problem
    Hello,

    I was doing some more testing with perfmon when I ran into
    a problem with the NMI watchdog code in 2.6.27-rc8.

    Since 2.6.20, it is possible to enable/disable the NMI watchdog
    on-the-fly via /proc/sys/kernel/nmi_watchdog. This is a nice option
    which avoids having to reboot the kernel.

    Enabling/disabling the NMI watchdog uses two internal functions
    enable_lapic_nmi_watchdog() and disable_lapic_nmi_watchdog().

    Enable_lapic_nmi_watchdog() uses a IPI handler to setup the
    APIC on each CPU. However, it turns out that this handler, namely,
    setup_apic_nmi_watchdog() relies on some explicit ordering constraint
    due to suspend/resume constraints as explained in the comment
    below:

    void setup_apic_nmi_watchdog(void *unused)
    {
    if (__get_cpu_var(wd_enabled))
    return;

    /* cheap hack to support suspend/resume */
    /* if cpu0 is not active neither should the other cpus */
    if (smp_processor_id() != 0 && atomic_read(&nmi_active) <= 0)
    return;

    switch (nmi_watchdog) {
    [snip]
    }

    Supposing watchdog was disabled via /proc, nmi_active = 0. Then if you
    re-enable, and if CPU0 is not the first to execute the IPI handler, then none
    of the other CPUS will re-enable their NMI watchdog timer. On a quad-core
    system, I have seen, for instance, 2 out of 4 with NMI watchdogs re-enabled.

    I am not an expert at suspend/resume. I am assuming there was a race condition
    there and that's why this code was added early on. The problem is that it now
    conflicts with the /proc option.

    It is not clear to me how this works during boot. Obviously the order
    is respected
    and all CPUs have their NMI watchdog enabled.

    Until I understand the suspend/resume issue, it is hard to provide a
    fix for this.

    Any comments?


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