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

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Message-ID: <87mygt9815.fsf@basil.nowhere.org>
Date:	Fri, 24 Oct 2008 20:12:38 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Mike Travis <travis@....com>
Cc:	Ingo Molnar <mingo@...e.hu>, Pavel Machek <pavel@...e.cz>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jack Steiner <steiner@....com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Richard Purdie <rpurdie@...ys.net>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] SGI X86 UV: Provide a System Activity Indicator	driver

Mike Travis <travis@....com> writes:
>
> I do use the timer mechanism (once per second) to update the scir state.
> Unfortunately, the state is *always* not idle during the timer callback
> (since we are actively executing), so I needed some way of knowing the
> idle state prior to the timer callback.

The idle thread always has pid 0. So if the timer sees current->pid == 0
it interrupted idle.

But sampling only once a second would be presumably not very accurate.
You could just check the per cpu statistics the kernel keeps
anyways in the timer.

I agree with Ingo that this doesn't belong in a idle notifier.

-Andi
-- 
ak@...ux.intel.com
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