linux-kernel - Re: [PATCH 2.6.21 review I] [11/25] x86: default to physical mode on hotplug CPU kernels

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Message-ID: <m1vei9ymkh.fsf@ebiederm.dsl.xmission.com>
Date:	Sun, 11 Feb 2007 04:13:02 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andi Kleen <ak@...e.de>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	"Li, Shaohua" <shaohua.li@...el.com>, patches@...-64.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2.6.21 review I] [11/25] x86: default to physical mode on hotplug CPU kernels

Andi Kleen <ak@...e.de> writes:

> From: Ingo Molnar <mingo@...e.hu>
>
> Default to physical mode on hotplug CPU kernels.  Furher simplify and clean up
> the APIC initialization code.

Where is the code that the subject describes?

I have two problems here.

- I don't see anything handling the hotplug case, and forcing us to
  physical mode.
- Ingo's other patch asserts that hotplug should be made to handle
  logical deliver mode.

With logical deliver mode the experimental evidence is that the
destination cpu is a hint, and you can arrive at a cpu that is not
in your cpu mask.  Now I only saw that problem on hyperthreaded cpus
but we didn't have the code enabled long enough to see it in other
cases.

Maybe if I can finish getting irq migration back into process context,
and someone verifies that the cpu disable in the hotplug path actually
disables the cpu/hyperthread instead of sitting in a hlt loop.  We
won't have a problem.  But broadcast ipis and irqs that don't go where
you tell them to are things we need to be very careful with.

Eric
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