[PATCH] x86: reorder mm_context_t to remove x86_64 alignment padding & so shrink mm_struct

[PATCH] x86: reorder mm_context_t to remove x86_64 alignment padding & so shrink mm_struct

[Richard Kennedy]

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@rsk.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

[tip:x86/urgent] x86: Reorder mm_context_t to remove x86_64 alignment padding and thus shrink mm_struct

[tip-bot for Richard Kennedy]

Commit-ID:  af6a25f0e1ec0265c267e6ee4513925eaba6d0ed
Gitweb:     http://git.kernel.org/tip/af6a25f0e1ec0265c267e6ee4513925eaba6d0ed
Author:     Richard Kennedy <richard@rsk.demon.co.uk>
AuthorDate: Tue, 24 May 2011 14:49:59 +0100
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Wed, 25 May 2011 16:16:41 +0200

x86: Reorder mm_context_t to remove x86_64 alignment padding and thus 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@rsk.demon.co.uk>
Cc: wilsons@start.ca
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Pekka Enberg <penberg@kernel.org>
Link: http://lkml.kernel.org/r/1306244999.1999.5.camel@castor.rsk
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 arch/x86/include/asm/mmu.h |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

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