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Commit a37e6e344910a43b9ebc2bbf29a029f5ea942598

Authored by Eric Dumazet
Committed by David S. Miller
1 parent 0c03eca3d9
Exists in smarc-l5.0.0_1.0.0-ga and in 5 other branches imx_3.10.17_1.0.1_ga, imx_3.10.53_1.1.0_ga, imx_3.14.28_1.0.0_ga, smarc-imx_3.10.53_1.1.0_ga, smarc-imx_3.14.28_1.0.0_ga

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>

Showing 2 changed files with 10 additions and 2 deletions Side-by-side Diff

... ... @@ -110,7 +110,7 @@
110 110 };
111 111  
112 112 extern u32 *dst_cow_metrics_generic(struct dst_entry *dst, unsigned long old);
113   -extern const u32 dst_default_metrics[RTAX_MAX];
  113 +extern const u32 dst_default_metrics[];
114 114  
115 115 #define DST_METRICS_READ_ONLY 0x1UL
116 116 #define __DST_METRICS_PTR(Y) \
... ... @@ -149,7 +149,15 @@
149 149 }
150 150 EXPORT_SYMBOL(dst_discard);
151 151  
152   -const u32 dst_default_metrics[RTAX_MAX];
  152 +const u32 dst_default_metrics[RTAX_MAX + 1] = {
  153 + /* This initializer is needed to force linker to place this variable
  154 + * into const section. Otherwise it might end into bss section.
  155 + * We really want to avoid false sharing on this variable, and catch
  156 + * any writes on it.
  157 + */
  158 + [RTAX_MAX] = 0xdeadbeef,
  159 +};
  160 +
153 161  
154 162 void *dst_alloc(struct dst_ops *ops, struct net_device *dev,
155 163 int initial_ref, int initial_obsolete, unsigned short flags)