Messages in this thread | | | Date | Mon, 25 Aug 2008 10:55:43 -0700 | From | Mike Travis <> | Subject | Re: [PATCH 3/6] x86_64 UV: Use blinking LED for heartbeat display |
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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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