linux-kernel - Re: [patch 00/47] Sparse irq rework

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Message-ID: <20101004063634.GA2230@elte.hu>
Date:	Mon, 4 Oct 2010 08:36:34 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	Russell King <linux@....linux.org.uk>,
	David Woodhouse <dwmw2@...radead.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Yinghai Lu <yinghai@...nel.org>,
	Grant Likely <grant.likely@...retlab.ca>
Subject: Re: [patch 00/47] Sparse irq rework


* Eric W. Biederman <ebiederm@...ssion.com> wrote:

> Looking at this objectively I see a bitmap sized to the formula 
> 32*NR_CPUS with NR_CPUS expected to double every 18-24 months. Which 
> means in a decade our worst case will be 2M not 64k.

On a 16 TB box? Noise.

> Can we please build this with scalable interfaces so we don't have to 
> do this again in a couple of years, and so we don't have to have 
> little machines paying the price for big machines, and so that big 
> machines aren't hamstrung because of data structures built for little 
> machines.

Bitmaps are rather scalable, we use them all around the place.

> Perhaps this just requires using ida instead of a bitmap.

Erm, IDA/IDR uses a bitmap ...

IDA is a dynamically allocated bitmap - but for something as critical as 
IRQs i'd rather like to see a preallocated bitmap and platform control 
over the max (nr_irqs). We dont want to allow crappy drivers to install 
irq 0x12345678 and blow up the bitmap size to dozens of MB, etc.

Bitmaps are simple - and that's a virtue.

Thanks,

	Ingo
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