Davide Libenzi a écrit :
+struct fd_map {
+ struct fd_map *next;
+ struct rcu_head rcu;
+ unsigned int base;
+ unsigned int size;
+ struct list_head slist;
+ struct list_head *slots;
+ unsigned int fdnext;
+ unsigned long *map;
+ void (*freecb)(void *, struct fd_map *);
+ void *freecb_priv;
+};
On x86_64 that would mean folowing offsets
struct fd_map {
struct fd_map *next; /* 0 */
struct rcu_head rcu; /* 8 */
unsigned int base; /* 0x18 */
unsigned int size; /* 0x1c */
struct list_head slist; /* 0x20 */
struct list_head *slots; /* 0x30 */
unsigned int fdnext; /* 0x38 */
unsigned long *map; /* 0x40 */
void (*freecb)(void *, struct fd_map *); /* 0x48 */
void *freecb_priv; /* 0x50 */
}; /* size 0x58 */
As L1_CACHE_BYTES is 64, two cache lines are necessary to get base,size,map
And a change of fdnext dirties one cache line that should be kept read only to
avoid false sharing.
I suggest to reorder to move rcu and next at the end (since they are seldom
used). Also move fdnext on a separate cache line on SMP
struct fd_map {
/*
* read mostly part
*/
unsigned int base; /* 0x00 */
unsigned int size; /* 0x04 */
struct list_head slist; /* 0x08 */
struct list_head *slots; /* 0x18 */
unsigned long *map; /* 0x28 */
void (*freecb)(void *, struct fd_map *); /* 0x30 */
void *freecb_priv; /* 0x38 */
/*
* written part on a separate cache line in SMP
*/
unsigned int fdnext ____cacheline_aligned_in_smp; /* 0x40 */
struct fd_map *next; /* 0x48 */
struct rcu_head rcu; /* 0x50 */
};
Thank you
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