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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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