From a37e6e344910a43b9ebc2bbf29a029f5ea942598 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Tue, 7 Aug 2012 10:55:45 +0000 Subject: [PATCH] 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 Cc: Ben Hutchings Signed-off-by: David S. Miller --- include/net/dst.h | 2 +- net/core/dst.c | 10 +++++++++- 2 files changed, 10 insertions(+), 2 deletions(-) diff --git a/include/net/dst.h b/include/net/dst.h index baf59789006..621e3513ef5 100644 --- a/include/net/dst.h +++ b/include/net/dst.h @@ -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) \ diff --git a/net/core/dst.c b/net/core/dst.c index 069d51d2941..56d63612e1e 100644 --- a/net/core/dst.c +++ b/net/core/dst.c @@ -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) -- 2.20.1