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

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Message-ID: <20080813095715.GB27074@atrey.karlin.mff.cuni.cz>
Date:	Wed, 13 Aug 2008 11:57:15 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Mike Travis <travis@....com>,
	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.

LED subsystem already has nice heartbeat trigger.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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