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

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Date:	Tue, 26 Aug 2008 13:53:48 -0700
From:	Mike Travis <travis@....com>
To:	Pavel Machek <pavel@...e.cz>
CC:	Ingo Molnar <mingo@...e.hu>,
	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

Pavel Machek 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.
> 
> LED subsystem already has nice heartbeat trigger.
> 									Pavel

>From Documentation/leds-class.txt:


Future Development
==================

At the moment, a trigger can't be created specifically for a single LED.
There are a number of cases where a trigger might only be mappable to a
particular LED (ACPI?). The addition of triggers provided by the LED driver
should cover this option and be possible to add without breaking the
current interface.

--
The SGI system has a set of leds per cpu, and the goal is that the leds
display heartbeat and "idle-ness" information specific to that cpu.  At
first glance I don't see this capability in the LED subsystem.  Am I
missing something?

Additionally, the 8 leds are written as one byte with each led being
full on or full off.  It seems to be a big overhead to support the led
class  since I'd have to kluge together some means of sharing the current
led register value [we really don't want to have to read the current reg
value before updating a single led.]

But I will definitely look at the led_heartbeat_function more closely.

Any advice gladly welcomed!

Thanks!
Mike
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