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net: force dst_default_metrics to const section 

While investigating on network performance problems, I found this little
gem :

$ nm -v vmlinux | grep -1 dst_default_metrics
ffffffff82736540 b busy.46605
ffffffff82736560 B dst_default_metrics
ffffffff82736598 b dst_busy_list

Apparently, declaring a const array without initializer put it in
(writeable) bss section, in middle of possibly often dirtied cache
lines.

Since we really want dst_default_metrics be const to avoid any possible
false sharing and catch any buggy writes, I force a null initializer.

ffffffff818a4c20 R dst_default_metrics

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2012-08-07 10:55:45 +00:00 committed by David S. Miller
parent 0c03eca3d9
commit a37e6e3449
2 changed files with 10 additions and 2 deletions
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2
include/net/dst.h
View File

@ -110,7 +110,7 @@ struct dst_entry {
};
extern u32 *dst_cow_metrics_generic(struct dst_entry *dst, unsigned long old);
extern const u32 dst_default_metrics[RTAX_MAX];
extern const u32 dst_default_metrics[];
#define DST_METRICS_READ_ONLY 0x1UL
#define __DST_METRICS_PTR(Y) \

10
net/core/dst.c
View File

@ -149,7 +149,15 @@ int dst_discard(struct sk_buff *skb)
}
EXPORT_SYMBOL(dst_discard);
const u32 dst_default_metrics[RTAX_MAX];
const u32 dst_default_metrics[RTAX_MAX + 1] = {
/* This initializer is needed to force linker to place this variable
* into const section. Otherwise it might end into bss section.
* We really want to avoid false sharing on this variable, and catch
* any writes on it.
*/
[RTAX_MAX] = 0xdeadbeef,
};
void *dst_alloc(struct dst_ops *ops, struct net_device *dev,
int initial_ref, int initial_obsolete, unsigned short flags)