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    SubjectRe: [PATCH 3/6] x86_64 UV: Use blinking LED for heartbeat display
    Ingo Molnar wrote:
    > * Mike Travis <travis@sgi.com> wrote:
    >
    >> +#ifdef CONFIG_CLOCKSOURCE_WATCHDOG
    >> +static void uv_display_heartbeat(void)
    >> +{
    >> + int cpu;
    >> +
    >> + uv_hub_info->led_heartbeat_count = nr_cpu_ids;
    >> +
    >> + for_each_online_cpu(cpu) {
    >> + struct uv_hub_info_s *hub = uv_cpu_hub_info(cpu);
    >> +
    >> + if (hub->led_heartbeat_count > 0) {
    >> + uv_set_led_bits_on(cpu, LED_CPU_BLINK,
    >> + LED_CPU_HEARTBEAT);
    >> + --hub->led_heartbeat_count;
    >> + }
    >
    > this too is a bad idea. Imagine 16K cores and assume that each such
    > iteration takes a few usecs (we write cross CPU) and you've got a
    > GHz-ish CPU. That can easily be _milliseconds_ of delay (or more) - and
    > in a function (the clocksource watchdog) that is all about precise
    > timings.
    >
    > It is also very non-preemptable.
    >
    > Why not have a separate per cpu kthread for this that does this in a
    > preemptable manner?
    >
    > Also, why not let each CPU's heartbeat be set in a hierarchy instead of
    > by _all_ CPUs. That way you get a nice constant-ish overhead instead of
    > the current crazy quadratic(nr_cpus) behavior. I.e. let each CPU be
    > monitored by its neighbor (cpu_id + 1), by it's second-order neighbor
    > (cpu_id + 2), third-order neighbor (cpu_id + 4), etc.
    >
    > That still gives pretty good coverage in practice while avoiding the
    > quadratic nature.
    >
    > Ingo

    Yes, I agree 100%. There was a trade off with various approaches but I
    was hoping for some feedback on alternate approaches (and thanks for that!)

    Mike


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