linux-kernel - [PATCH] x86: reorder mm_context_t to remove x86_64 alignment padding & so shrink mm

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Date:	Tue, 24 May 2011 14:49:59 +0100
From:	Richard Kennedy <richard@....demon.co.uk>
To:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>, wilsons@...rt.ca
Subject: [PATCH] x86: reorder mm_context_t to remove x86_64 alignment
 padding & so shrink mm_struct

Reorder mm_context_t to remove alignment padding on 64 bit builds
shrinking its size from 64 to 56 bytes.
    
This allows mm_struct to shrink from 840 to 832 bytes, so using one
fewer cache lines, and getting more objects per slab when using slub.
    
    
slabinfo mm_struct reports
before :-
    
    Sizes (bytes)     Slabs
    -----------------------------------
    Object :     840  Total  :       7
    SlabObj:     896  Full   :       1
    SlabSiz:   16384  Partial:       4
    Loss   :      56  CpuSlab:       2
    Align  :      64  Objects:      18
    
after :-

    Sizes (bytes)     Slabs
    ----------------------------------
    Object :     832  Total  :       7
    SlabObj:     832  Full   :       1
    SlabSiz:   16384  Partial:       4
    Loss   :       0  CpuSlab:       2
    Align  :      64  Objects:      19
    
Signed-off-by: Richard Kennedy <richard@....demon.co.uk>

---
patch against v2.6.39
compiled & tested on x86_64.

regards
Richard



diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h
index aeff3e8..5f55e69 100644
--- a/arch/x86/include/asm/mmu.h
+++ b/arch/x86/include/asm/mmu.h
@@ -11,14 +11,14 @@
 typedef struct {
 	void *ldt;
 	int size;
-	struct mutex lock;
-	void *vdso;
 
 #ifdef CONFIG_X86_64
 	/* True if mm supports a task running in 32 bit compatibility mode. */
 	unsigned short ia32_compat;
 #endif
 
+	struct mutex lock;
+	void *vdso;
 } mm_context_t;
 
 #ifdef CONFIG_SMP


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