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

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Date:	Mon, 25 Aug 2008 10:55:43 -0700
From:	Mike Travis <travis@....com>
To:	Ingo Molnar <mingo@...e.hu>
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

Ingo Molnar wrote:
> * 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

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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