linux-kernel - Re: [patch] genapic: optimize & fix APIC mode setup

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Message-ID: <20061113190452.GA29109@elte.hu>
Date:	Mon, 13 Nov 2006 20:04:52 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
Cc:	Andi Kleen <ak@...e.de>, Andrew Morton <akpm@...l.org>,
	linux-kernel@...r.kernel.org, ashok.raj@...el.com
Subject: Re: [patch] genapic: optimize & fix APIC mode setup


* Siddha, Suresh B <suresh.b.siddha@...el.com> wrote:

> On Mon, Nov 13, 2006 at 07:42:56PM +0100, Ingo Molnar wrote:
> > * Siddha, Suresh B <suresh.b.siddha@...el.com> wrote:
> > > Not really. That chipset belongs to a MP platform and with your 
> > > proposed patch, we will endup using clustered APIC mode and will hit 
> > > the issue(in the presence of cpu hotplug) mentioned in that URL.
> > 
> > hm, why does it end up in clustered mode? Cluster mode should only 
> > trigger if the APIC IDs go beyond 16.
> 
> go beyond '8' not 16. With Dual-core+HT these MP platforms will have 
> 16 logical cpus.

yes - then the APIC ids will go beyond 15.

> > but i'd be fine with never going into cluster mode, instead always 
> > using physical flat mode when having more than 8 APICs (independent 
> > of the presence of CPU hotplug). On small systems, logical flat mode 
> > is what is the best-tested variant (it's also slightly faster).
> 
> Ok.

ok, that's really good. Is there any 'weird' platform that you are aware 
of that absolutely needs clustered APIC mode (because it has no physical 
delivery mode or something)?

	Ingo
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