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

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Date:	Sat, 11 Nov 2006 16:27:57 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <ak@...e.de>
Cc:	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	ashok.raj@...el.com
Subject: Re: [patch] genapic: optimize & fix APIC mode setup

* Andi Kleen <ak@...e.de> wrote:

> On Saturday 11 November 2006 16:14, Ingo Molnar wrote:
> > From: Ingo Molnar <mingo@...e.hu>
> > 
> > this patch fixes a couple of inconsistencies/problems i found while 
> > reviewing the x86_64 genapic code (when i was chasing mysterious eth0 
> > timeouts that would only trigger if CPU_HOTPLUG is enabled):
> > 
> >  - AMD systems defaulted to the slower flat-physical mode instead
> >    of the flat-logical mode. The only restriction on AMD systems
> >    is that they should not use clustered APIC mode.
> 
> This will open a race on CPU hotunplug unfortunately (common for multi 
> core suspend)

Note that i386 still defaults to logical flat mode, so whatever hotplug 
CPU races there are, they need to be fixed! Given how rare CPU hotplug 
systems are i have no problem with having these races in the kernel for 
a while until it's fixed.

Also, distro kernels enable CPU_HOTPLUG frequently. It is just ugly 
beyond recognition to switch the programming of the IRQ hardware on 
non-hotplug hardware just because a mostly-software feature (hotplug) is 
enabled ...

if hotplug breaks suspend then fix it, dont hack it around (on one 
platform) by slowing down the system [and causing other problems] ...

	Ingo
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