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    SubjectRe: [PATCH 3/4] nmi_watchdog: fallback to software events when no hardware pmu detected
    On Fri, Feb 12, 2010 at 05:19:20PM -0500, Don Zickus wrote:

    > diff --git a/kernel/nmi_watchdog.c b/kernel/nmi_watchdog.c
    > index 73c1954..4f23505 100644
    > --- a/kernel/nmi_watchdog.c
    > +++ b/kernel/nmi_watchdog.c
    > @@ -166,8 +166,12 @@ cpu_callback(struct notifier_block *nfb, unsigned long action, void *hcpu)
    > wd_attr.sample_period = hw_nmi_get_sample_period();
    > event = perf_event_create_kernel_counter(&wd_attr, hotcpu, -1, wd_overflow);
    > if (IS_ERR(event)) {
    > - printk(KERN_ERR "nmi watchdog failed to create perf event on %i: %p\n", hotcpu, event);
    > - return NOTIFY_BAD;
    > + wd_attr.type = PERF_TYPE_SOFTWARE;
    > + event = perf_event_create_kernel_counter(&wd_attr, hotcpu, -1, wd_overflow);

    Here you don't explicitly set wd_attr.config or wd_attr.sample_period
    for the software event. So PERF_COUNT_HW_CPU_CYCLES (which is 0)
    becomes PERF_COUNT_SW_CPU_CLOCK (also 0). Which is either a happy
    accident or really really subtle. :) I suggest you either set
    wd_attr.config or add a comment pointing out that you are using the
    fact that PERF_COUNT_HW_CPU_CYCLES == PERF_COUNT_SW_CPU_CLOCK.

    Also you don't explicitly set wd_attr.sample_period, so it will be
    whatever hw_nmi_get_sample_period() returned, except now measured in
    nanoseconds rather than (presumably) cpu clock cycles. Are you aiming
    for 1 interrupt per second? If so you should set .sample_period to
    NSEC_PER_SEC.

    Paul.


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