linux-kernel - Re: [PATCH 1/1] SGI X86 UV: Provide a System Activity Indicator driver

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Message-ID: <20080928194201.GA1310@ucw.cz>
Date:	Sun, 28 Sep 2008 21:42:01 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	Mike Travis <travis@....com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>, rpurdie@...ys.net,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/1] SGI X86 UV: Provide a System Activity Indicator
	driver


> Another relevant point is that I will be adding a bit more functionality
> to disable the timer interrupt on truly "idle" cpus (like have been idle
> for some amount of seconds).  We would then use the "exit from idle"
> callback to reestablish the timer interrupt.  [This would allow them to
> enter power down states if appropriate.]

Should you look at nohz instead of reinventing it? 

> > As i suggested in my previous mail about this topic, a low-frequency 
> > sampling method should be used instead, to indicate system status. I 
> > thought the leds drivers have all that in place already.
> 
> It is low frequency (once per second), this is just setting what's to
> be sampled.
> 
> As I mentioned, this is not for LED displays (human readable), it's for the
> system controller to monitor how all parts of the system are running, and
> this one is just the cpu parts.  The LED driver approach would have me
> registering 4096 led devices, with all their callbacks, 4096 strings saying
> "LED0001", etc., and I still cannot associate a specific register bit
> (AKA LED if that's what it was), with a specific cpu using the LED driver.
> 
> The LED driver is fine for a couple of blinking lights indicating overall
> system activity, disk activity, etc.  (Btw, I did not see a network trigger,
> or a paging trigger, or an out of memory trigger, or some other things that
> might be useful for real time monitoring of the system.)

...so add them...

> But the LED driver has way more overhead than needed for this simple application.
> 

So overhead from led driver is not okay, while overhead from messing
with idle loop is okay? Interesting...
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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