linux-kernel - Re: [tip:x86/apic] x86/apic: Try to spread IRQ vectors to different priority levels

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Message-ID: <4FE2071B.5040602@zytor.com>
Date:	Wed, 20 Jun 2012 10:23:39 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	mingo@...nel.org, hpa@...or.com, agordeev@...hat.com,
	linux-kernel@...r.kernel.org, yinghai@...nel.org,
	suresh.b.siddha@...el.com, tglx@...utronix.de
CC:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/apic] x86/apic: Try to spread IRQ vectors to different
 priority levels

On 06/08/2012 07:50 AM, tip-bot for Alexander Gordeev wrote:
> Commit-ID:  1bccd58bfffc5a677051937b332b71f0686187c1
> Gitweb:     http://git.kernel.org/tip/1bccd58bfffc5a677051937b332b71f0686187c1
> Author:     Alexander Gordeev <agordeev@...hat.com>
> AuthorDate: Thu, 7 Jun 2012 15:15:15 +0200
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Fri, 8 Jun 2012 11:44:28 +0200
> 
> x86/apic: Try to spread IRQ vectors to different priority levels
> 
> When assigning a new vector it is primarially done by adding 8
> to the previously given out vector number. Hence, two
> consequently allocated vector numbers would likely fall into the
> same priority level. Try to spread vector numbers to different
> priority levels better by changing the step from 8 to 16.
> 

OK, stupid question: WHY?

In general, in Linux the random prioritization is actually a negative.
The only reason for the spreading by 8 is because of bugs/misfeatures in
old APIC implementations which made them handle more than two interrupts
per priority level rather inefficiently.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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