linux-kernel - Re: x86: unify genapic code, unify subarchitectures, remove old subarchitecture code

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Message-ID: <20090129222956.GA1668@elte.hu>
Date:	Thu, 29 Jan 2009 23:29:56 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: x86: unify genapic code, unify subarchitectures, remove old
	subarchitecture code

* Andi Kleen <andi@...stfloor.org> wrote:

> Also bigsmp is kind of obsolete too, it could be probably
> merged with default with very tiny impact because it's not
> all that different.

i dont think you have understood the driving principle behind this 
restructuring and unification.

The whole _point_ of the apic driver infrastructure is to use it for 
dissimilar APIC modes.

In the future we'll create _more_ of them, not less - so that we can 
remove runtime conditionals from the drivers and that we can use the APIC 
driver for the hardware that is the most efficient.

For example, the apic_default driver (which is used if all APIC IDs are 
between 0 and 7 - which is the majority of current hardware), is more 
efficient at sending IPIs, because it can send a broadcast IPI (used for 
TLB flushes for example) as a single IPI.

The apic_bigsmp driver on the other hand has to use a 'serialized' 
sequence of IPIs, to each CPU separately. That is less efficient, 
especially for something as performance-sensitive as TLB flushes.

To "unify" the ability of apic_bigsmp to deal with APIC IDs >= 8 with the 
more efficient logical addressing mode of apc_default is possible but ugly 
- we'd have to add runtime checks and flip between the two methods. What 
So what you call "very tiny impact" is in reality unnecessary and 
avoidable overhead.

	Ingo
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