net: force dst_default_metrics to const section (a37e6e34) · Commits · 戴 / test

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>

include/net/dst.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -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) \

net/core/dst.c

+9 −1

Original line number	Diff line number	Diff line
		@@ -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)

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