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Subject[PATCH] percpu: Introduce DEFINE_PER_CPU_PAGE_ALIGNED() macro
While examining holes in percpu section I found this :

c05f5000 D per_cpu__current_task
c05f5000 D __per_cpu_start
c05f5004 D per_cpu__cpu_number
c05f5008 D per_cpu__irq_regs
c05f500c d per_cpu__cpu_devices
c05f5040 D per_cpu__cyc2ns

<Big Hole of about 4000 bytes>

c05f6000 d per_cpu__cpuid4_info
c05f6004 d per_cpu__cache_kobject
c05f6008 d per_cpu__index_kobject

<Big Hole of about 4000 bytes>

c05f7000 D per_cpu__gdt_page

This is because gdt_page is a percpu variable, defined with
a page alignement, and linker is doing its job, two times because of .o
nesting in the build process.

I introduced a new macro DEFINE_PER_CPU_PAGE_ALIGNED() to avoid
wasting this space. All page aligned variables (only one at this time)
are put in a separate
subsection .data.percpu.page_aligned, at the very begining of percpu zone.

Before patch , on a x86_32 machine :

# size -A vmlinux.old vmlinux | grep percpu
.data.percpu 30232 3227471872
.data.percpu 22168 3227471872

Thats 8064 bytes saved for each CPU.

Patch against x86.git/latest

Note : This will collide with Christoph efforts to put pda at offset 0
of percpu zone...

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>

arch/x86/kernel/cpu/common.c | 2 +-
arch/x86/kernel/vmlinux_32.lds.S | 1 +
include/asm-generic/vmlinux.lds.h | 1 +
include/linux/percpu.h | 7 +++++++
4 files changed, 10 insertions(+), 1 deletion(-)


diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index b186047..c665dfc 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -21,7 +21,7 @@

#include "cpu.h"

-DEFINE_PER_CPU(struct gdt_page, gdt_page) = { .gdt = {
+DEFINE_PER_CPU_PAGE_ALIGNED(struct gdt_page, gdt_page) = { .gdt = {
[GDT_ENTRY_KERNEL_CS] = { { { 0x0000ffff, 0x00cf9a00 } } },
[GDT_ENTRY_KERNEL_DS] = { { { 0x0000ffff, 0x00cf9200 } } },
[GDT_ENTRY_DEFAULT_USER_CS] = { { { 0x0000ffff, 0x00cffa00 } } },
diff --git a/arch/x86/kernel/vmlinux_32.lds.S b/arch/x86/kernel/vmlinux_32.lds.S
index ce5ed08..0f7c29a 100644
--- a/arch/x86/kernel/vmlinux_32.lds.S
+++ b/arch/x86/kernel/vmlinux_32.lds.S
@@ -189,6 +189,7 @@ SECTIONS
. = ALIGN(PAGE_SIZE);
.data.percpu : AT(ADDR(.data.percpu) - LOAD_OFFSET) {
__per_cpu_start = .;
+ *(.data.percpu.page_aligned)
*(.data.percpu)
*(.data.percpu.shared_aligned)
__per_cpu_end = .;
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index f054778..69e5c11 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -348,6 +348,7 @@
. = ALIGN(align); \
__per_cpu_start = .; \
.data.percpu : AT(ADDR(.data.percpu) - LOAD_OFFSET) { \
+ *(.data.percpu.page_aligned) \
*(.data.percpu) \
*(.data.percpu.shared_aligned) \
} \
diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index 1ac9697..1c0cc0d 100644
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -18,12 +18,19 @@
__attribute__((__section__(".data.percpu.shared_aligned"))) \
PER_CPU_ATTRIBUTES __typeof__(type) per_cpu__##name \
____cacheline_aligned_in_smp
+
+#define DEFINE_PER_CPU_PAGE_ALIGNED(type, name) \
+ __attribute__((__section__(".data.percpu.page_aligned"))) \
+ PER_CPU_ATTRIBUTES __typeof__(type) per_cpu__##name
#else
#define DEFINE_PER_CPU(type, name) \
PER_CPU_ATTRIBUTES __typeof__(type) per_cpu__##name

#define DEFINE_PER_CPU_SHARED_ALIGNED(type, name) \
DEFINE_PER_CPU(type, name)
+
+#define DEFINE_PER_CPU_PAGE_ALIGNED(type, name) \
+ DEFINE_PER_CPU(type, name)
#endif

#define EXPORT_PER_CPU_SYMBOL(var) EXPORT_SYMBOL(per_cpu__##var)
\
 
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