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Subject[PATCH][MM]: Save some bytes in struct mm_struct
Beware: just compile tested!

Before:
[acme@newtoy net-2.6.20]$ pahole --cacheline 32 kernel/sched.o mm_struct

/* include2/asm/processor.h:542 */
struct mm_struct {
struct vm_area_struct * mmap; /* 0 4 */
struct rb_root mm_rb; /* 4 4 */
struct vm_area_struct * mmap_cache; /* 8 4 */
long unsigned int (*get_unmapped_area)(); /* 12 4 */
void (*unmap_area)(); /* 16 4 */
long unsigned int mmap_base; /* 20 4 */
long unsigned int task_size; /* 24 4 */
long unsigned int cached_hole_size; /* 28 4 */
/* ---------- cacheline 1 boundary ---------- */
long unsigned int free_area_cache; /* 32 4 */
pgd_t * pgd; /* 36 4 */
atomic_t mm_users; /* 40 4 */
atomic_t mm_count; /* 44 4 */
int map_count; /* 48 4 */
struct rw_semaphore mmap_sem; /* 52 64 */
spinlock_t page_table_lock; /* 116 40 */
struct list_head mmlist; /* 156 8 */
mm_counter_t _file_rss; /* 164 4 */
mm_counter_t _anon_rss; /* 168 4 */
long unsigned int hiwater_rss; /* 172 4 */
long unsigned int hiwater_vm; /* 176 4 */
long unsigned int total_vm; /* 180 4 */
long unsigned int locked_vm; /* 184 4 */
long unsigned int shared_vm; /* 188 4 */
/* ---------- cacheline 6 boundary ---------- */
long unsigned int exec_vm; /* 192 4 */
long unsigned int stack_vm; /* 196 4 */
long unsigned int reserved_vm; /* 200 4 */
long unsigned int def_flags; /* 204 4 */
long unsigned int nr_ptes; /* 208 4 */
long unsigned int start_code; /* 212 4 */
long unsigned int end_code; /* 216 4 */
long unsigned int start_data; /* 220 4 */
/* ---------- cacheline 7 boundary ---------- */
long unsigned int end_data; /* 224 4 */
long unsigned int start_brk; /* 228 4 */
long unsigned int brk; /* 232 4 */
long unsigned int start_stack; /* 236 4 */
long unsigned int arg_start; /* 240 4 */
long unsigned int arg_end; /* 244 4 */
long unsigned int env_start; /* 248 4 */
long unsigned int env_end; /* 252 4 */
/* ---------- cacheline 8 boundary ---------- */
long unsigned int saved_auxv[44]; /* 256 176 */
unsigned int dumpable:2; /* 432 4 */
cpumask_t cpu_vm_mask; /* 436 4 */
mm_context_t context; /* 440 68 */
long unsigned int swap_token_time; /* 508 4 */
/* ---------- cacheline 16 boundary ---------- */
char recent_pagein; /* 512 1 */

/* XXX 3 bytes hole, try to pack */

int core_waiters; /* 516 4 */
struct completion * core_startup_done; /* 520 4 */
struct completion core_done; /* 524 52 */
rwlock_t ioctx_list_lock; /* 576 36 */
struct kioctx * ioctx_list; /* 612 4 */
}; /* size: 616, sum members: 613, holes: 1, sum holes: 3, cachelines: 20,
last cacheline: 8 bytes */

After:

[acme@newtoy net-2.6.20]$ pahole --cacheline 32 kernel/sched.o mm_struct
/* include2/asm/processor.h:542 */
struct mm_struct {
struct vm_area_struct * mmap; /* 0 4 */
struct rb_root mm_rb; /* 4 4 */
struct vm_area_struct * mmap_cache; /* 8 4 */
long unsigned int (*get_unmapped_area)(); /* 12 4 */
void (*unmap_area)(); /* 16 4 */
long unsigned int mmap_base; /* 20 4 */
long unsigned int task_size; /* 24 4 */
long unsigned int cached_hole_size; /* 28 4 */
/* ---------- cacheline 1 boundary ---------- */
long unsigned int free_area_cache; /* 32 4 */
pgd_t * pgd; /* 36 4 */
atomic_t mm_users; /* 40 4 */
atomic_t mm_count; /* 44 4 */
int map_count; /* 48 4 */
struct rw_semaphore mmap_sem; /* 52 64 */
spinlock_t page_table_lock; /* 116 40 */
struct list_head mmlist; /* 156 8 */
mm_counter_t _file_rss; /* 164 4 */
mm_counter_t _anon_rss; /* 168 4 */
long unsigned int hiwater_rss; /* 172 4 */
long unsigned int hiwater_vm; /* 176 4 */
long unsigned int total_vm; /* 180 4 */
long unsigned int locked_vm; /* 184 4 */
long unsigned int shared_vm; /* 188 4 */
/* ---------- cacheline 6 boundary ---------- */
long unsigned int exec_vm; /* 192 4 */
long unsigned int stack_vm; /* 196 4 */
long unsigned int reserved_vm; /* 200 4 */
long unsigned int def_flags; /* 204 4 */
long unsigned int nr_ptes; /* 208 4 */
long unsigned int start_code; /* 212 4 */
long unsigned int end_code; /* 216 4 */
long unsigned int start_data; /* 220 4 */
/* ---------- cacheline 7 boundary ---------- */
long unsigned int end_data; /* 224 4 */
long unsigned int start_brk; /* 228 4 */
long unsigned int brk; /* 232 4 */
long unsigned int start_stack; /* 236 4 */
long unsigned int arg_start; /* 240 4 */
long unsigned int arg_end; /* 244 4 */
long unsigned int env_start; /* 248 4 */
long unsigned int env_end; /* 252 4 */
/* ---------- cacheline 8 boundary ---------- */
long unsigned int saved_auxv[44]; /* 256 176 */
cpumask_t cpu_vm_mask; /* 432 4 */
mm_context_t context; /* 436 68 */
long unsigned int swap_token_time; /* 504 4 */
char recent_pagein; /* 508 1 */
unsigned char dumpable:2; /* 509 1 */

/* XXX 2 bytes hole, try to pack */

int core_waiters; /* 512 4 */
struct completion * core_startup_done; /* 516 4 */
struct completion core_done; /* 520 52 */
rwlock_t ioctx_list_lock; /* 572 36 */
struct kioctx * ioctx_list; /* 608 4 */
}; /* size: 612, sum members: 610, holes: 1, sum holes: 2, cachelines: 20,
last cacheline: 4 bytes */

[acme@newtoy net-2.6.20]$ codiff -V /tmp/sched.o.before kernel/sched.o
/pub/scm/linux/kernel/git/acme/net-2.6.20/kernel/sched.c:
struct mm_struct | -4
dumpable:2;
from: unsigned int /* 432(30) 4(2) */
to: unsigned char /* 509(6) 1(2) */
< SNIP other offset changes >
1 struct changed
[acme@newtoy net-2.6.20]$

I'm not aware of any problem about using 2 byte wide bitfields where previously
a 4 byte wide one was, holler if there is any, I wouldn't be surprised,
bitfields are things from hell.

For the curious, 432(30) means: at offset 432 from the struct start, at offset 30
in the bitfield (yeah, it comes backwards, hellish, huh?) ditto for 509(6), while
4(2) and 1(2) means "struct field size(bitfield size)".

Now we have a 2 bytes hole and are using only 4 bytes of the last 32
bytes cacheline, any takers? :-)

Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
---
sched.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index eafe4a7..a79d1f5 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -338,7 +338,6 @@ struct mm_struct {

unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */

- unsigned dumpable:2;
cpumask_t cpu_vm_mask;

/* Architecture-specific MM context */
@@ -348,6 +347,8 @@ struct mm_struct {
unsigned long swap_token_time;
char recent_pagein;

+ unsigned char dumpable:2;
+
/* coredumping support */
int core_waiters;
struct completion *core_startup_done, core_done;
-
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