linux-kernel - Re: [PATCH] x86_64: remove never used nodenumer in pda

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Date:	Mon, 18 Feb 2008 13:23:11 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Yinghai Lu <Yinghai.Lu@....COM>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mike Travis <travis@....com>, lameter@....com
Subject: Re: [PATCH] x86_64: remove never used nodenumer in pda

Yinghai Lu <Yinghai.Lu@....COM> writes:

> we don't need copy too. already have x86_cpu_to_node_map

That's a regression (probably from Mike's patches?). Until recently it was 
used.

The reason the node number was put in there is that it generates far
shorter code to just fetch the local node number from the PDA than to
first go through a array lookup from the cpu number. It also saves a
costly cache line miss on the array if you're unlucky.

It is far better to fix it than to remove it.

I know Mike/Christoph want to get rid of the PDA and make per cpu data 
as efficient as the PDA. If that happens the right fix is to create
a new per CPU data variable for the node number again.

Here's a quick patch (tested on kvm with numa emulation only) 

It should be ok because PDA is set up early and
the early node is always 0 and there is a 0 in there
at early boot.

Saves about 1.6k of text on a vmlinux here.

Signed-off-by: Andi Kleen <ak@...e.de>

Index: linux/include/asm-x86/topology.h
===================================================================
--- linux.orig/include/asm-x86/topology.h
+++ linux/include/asm-x86/topology.h
@@ -34,11 +34,12 @@
 extern int cpu_to_node_map[];

 #else
+#include <asm/pda.h>
 DECLARE_PER_CPU(int, x86_cpu_to_node_map);
 extern int x86_cpu_to_node_map_init[];
 extern void *x86_cpu_to_node_map_early_ptr;
 /* Returns the number of the current Node. */
-#define numa_node_id()		(early_cpu_to_node(raw_smp_processor_id()))
+#define numa_node_id()		(read_pda(nodenumber))
 #endif

 extern cpumask_t node_to_cpumask_map[];

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