Arnaldo Carvalho de Melo a écrit :
Hi,
I've been working on some DWARF2 utilities and one of them,
pahole (Poke-a-Hole) can be used to find holes due to alignment rules
in structs, the full output of:
[EMAIL PROTECTED] net-2.6]$ pahole net/ipv4/tcp.o
is available at:
http://oops.merseine.nu:81/acme/net.ipv4.tcp.o.pahole
Just to show what we can find with this tool here is the layout
of struct net_device, that barring any cacheline locality optimization
has 4 bytes to harvest, David, do you think reordering those fields to
get 4 byts back is ok?
I just want to bring your attention this net_device structure was re-ordered
(by me :)) so that separate cache lines are used on SMP machines.
If you select CONFIG_SMP , you'll probably notice far more holes. But it was a
feature, not lazyness.
We can probably move some fields, but very carefully :)
[EMAIL PROTECTED] net-2.6]$ pahole net/ipv4/tcp.o net_device
/* /pub/scm/linux/kernel/git/acme/net-2.6/include/linux/skbuff.h:87 */
struct net_device {
char name[16]; /* 0 16 */
struct hlist_node name_hlist; /* 16 8 */
long unsigned int mem_end; /* 24 4 */
long unsigned int mem_start; /* 28 4 */
long unsigned int base_addr; /* 32 4 */
unsigned int irq; /* 36 4 */
unsigned char if_port; /* 40 1 */
unsigned char dma; /* 41 1 */
/* XXX 2 bytes hole, try to pack */
long unsigned int state; /* 44 4 */
struct net_device * next; /* 48 4 */
int (*init)(); /* 52 4 */
long unsigned int features; /* 56 4 */
struct net_device * next_sched; /* 60 4 */
int ifindex; /* 64 4 */
int iflink; /* 68 4 */
struct net_device_stats * (*get_stats)(); /* 72 4 */
const struct iw_handler_def * wireless_handlers; /* 76 4 */
struct iw_public_data * wireless_data; /* 80 4 */
const struct ethtool_ops * ethtool_ops; /* 84 4 */
unsigned int flags; /* 88 4 */
short unsigned int gflags; /* 92 2 */
short unsigned int priv_flags; /* 94 2 */
short unsigned int padded; /* 96 2 */
unsigned char operstate; /* 98 1 */
unsigned char link_mode; /* 99 1 */
unsigned int mtu; /* 100 4 */
short unsigned int type; /* 104 2 */
short unsigned int hard_header_len; /* 106 2 */
struct net_device * master; /* 108 4 */
unsigned char perm_addr[32]; /* 112 32 */
unsigned char addr_len; /* 144 1 */
/* XXX 1 bytes hole, try to pack */
short unsigned int dev_id; /* 146 2 */
struct dev_mc_list * mc_list; /* 148 4 */
int mc_count; /* 152 4 */
int promiscuity; /* 156 4 */
int allmulti; /* 160 4 */
void * atalk_ptr; /* 164 4 */
void * ip_ptr; /* 168 4 */
void * dn_ptr; /* 172 4 */
void * ip6_ptr; /* 176 4 */
void * ec_ptr; /* 180 4 */
void * ax25_ptr; /* 184 4 */
struct list_head poll_list; /* 188 8 */
int (*poll)(); /* 196 4 */
int quota; /* 200 4 */
int weight; /* 204 4 */
long unsigned int last_rx; /* 208 4 */
unsigned char dev_addr[32]; /* 212 32 */
unsigned char broadcast[32]; /* 244 32 */
spinlock_t queue_lock; /* 276 0 */
struct Qdisc * qdisc; /* 276 4 */
struct Qdisc * qdisc_sleeping; /* 280 4 */
struct list_head qdisc_list; /* 284 8 */
long unsigned int tx_queue_len; /* 292 4 */
struct sk_buff * gso_skb; /* 296 4 */
spinlock_t ingress_lock; /* 300 0 */
struct Qdisc * qdisc_ingress; /* 300 4 */
spinlock_t _xmit_lock; /* 304 0 */
int xmit_lock_owner; /* 304 4 */
void * priv; /* 308 4 */
int (*hard_start_xmit)(); /* 312 4 */
long unsigned int trans_start; /* 316 4 */
int watchdog_timeo; /* 320 4 */
struct timer_list watchdog_timer; /* 324 24 */
atomic_t refcnt; /* 348 4 */
struct list_head todo_list; /* 352 8 */
struct hlist_node index_hlist; /* 360 8 */
enum reg_state; /* 368 4 */
void (*uninit)(); /* 372 4 */
void (*destructor)(); /* 376 4 */
int (*open)(); /* 380 4 */
int (*stop)(); /* 384 4 */
int (*hard_header)(); /* 388 4 */
int (*rebuild_header)(); /* 392 4 */
void (*set_multicast_list)(); /* 396 4 */
int (*set_mac_address)(); /* 400 4 */
int (*do_ioctl)(); /* 404 4 */
int (*set_config)(); /* 408 4 */
int (*hard_header_cache)(); /* 412 4 */
void (*header_cache_update)(); /* 416 4 */
int (*change_mtu)(); /* 420 4 */
void (*tx_timeout)(); /* 424 4 */
void (*vlan_rx_register)(); /* 428 4 */
void (*vlan_rx_add_vid)(); /* 432 4 */
void (*vlan_rx_kill_vid)(); /* 436 4 */
int (*hard_header_parse)(); /* 440 4 */
int (*neigh_setup)(); /* 444 4 */
struct net_bridge_port * br_port; /* 448 4 */
struct class_device class_dev; /* 452 144 */
struct attribute_group * sysfs_groups[3]; /* 596 12 */
}; /* size: 608, sum members: 605, holes: 2, sum holes: 3 */
The git repo for pahole & the other dwarves is at:
git://git.kernel.org/pub/scm/linux/kernel/git/acme/pahole.git
For those wanting to just look at the cset comments (best
documentation available so far ;)):
http://www.kernel.org/git/?p=linux/kernel/git/acme/pahole.git;a=summary
Best Regards,
- Arnaldo
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