lkml.org 
[lkml]   [2008]   [Aug]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 2/6] x86_64 UV: Use LED to indicate CPU is active

* 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


\
 
 \ /
  Last update: 2008-08-11 17:55    [W:0.092 / U:10.356 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site