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Date: Mon, 11 Aug 2008 17:53:18 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Mike Travis <travis@sgi.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Jack Steiner <steiner@sgi.com>,
       linux-kernel@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
       Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 2/6] x86_64 UV: Use LED to indicate CPU is active
Message-ID: <20080811155318.GB4524@elte.hu>
References: <20080808005639.505189000@polaris-admin.engr.sgi.com> <20080808005649.986660000@polaris-admin.engr.sgi.com>
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* Mike Travis <travis@sgi.com> wrote:

> +/*
> + * Illuminate "activity" LED when CPU is going "active",
> + * extinguish when going "idle".
> + */
> +static int uv_idle(struct notifier_block *nfb, unsigned long action, void *junk)
> +{
> +	if (action == IDLE_START)
> +		uv_set_led_bits(0, LED_CPU_ACTIVITY);
> +
> +	else if (action == IDLE_END)
> +		uv_set_led_bits(LED_CPU_ACTIVITY, LED_CPU_ACTIVITY);
> +
> +	return NOTIFY_OK;
> +}
> +
> +static struct notifier_block uv_idle_notifier = {
> +	.notifier_call = uv_idle,
> +};
>  
> +/*
> + * Initialize idle led callback function
> + */
> +static __init void uv_init_led_idle_display(void)
> +{
> +	/* initialize timer for activity monitor */
> +	idle_notifier_register(&uv_idle_notifier);
> +}

hm, i think this is a bad idea. While putting it into the go-to-idle 
codepath probably doesnt matter, putting anything into the idle wakeup 
path increases latency of a rather critical codepath. The MMR write that 
the LED driver is using will go out to the local bus, which, even if 
it's POST-ed, if it's done frequently enough will be the cost of an IO 
cycle.

No human needs to know the LED status at _that_ frequency anyway, so 
it's quite pointless as well.

A much better (and faster) approach would be to sample the utilization 
of the CPU and indicate that via the LED(s).

	Ingo