linux-kernel - Re: [PATCH 3/6] x86_64 UV: Use blinking LED for heartbeat display

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Date:	Mon, 11 Aug 2008 18:02:35 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mike Travis <travis@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 3/6] x86_64 UV: Use blinking LED for heartbeat display

* Mike Travis <travis@....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
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