dynamic_debug: add jump label support

Although dynamic debug is often only used for debug builds, sometimes
its enabled for production builds as well.  Minimize its impact by using
jump labels.  This reduces the text section by 7000+ bytes in the kernel
image below.  It does increase data, but this should only be referenced
when changing the direction of the branches, and hence usually not in
cache.

     text     data     bss       dec     hex  filename
  8194852  4879776  925696  14000324  d5a0c4  vmlinux.pre
  8187337  4960224  925696  14073257  d6bda9  vmlinux.post

Link: http://lkml.kernel.org/r/d165b465e8c89bc582d973758d40be44c33f018b.1467837322.git.jbaron@akamai.com
Signed-off-by: Jason Baron <jbaron@akamai.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Joe Perches <joe@perches.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index fe42b6ec3f0c..da796e2dc4f5 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -188,6 +188,13 @@ static int ddebug_change(const struct ddebug_query *query,
                         newflags = (dp->flags & mask) | flags;
                         if (newflags == dp->flags)
                                 continue;
+#ifdef HAVE_JUMP_LABEL
+                        if (dp->flags & _DPRINTK_FLAGS_PRINT) {
+                                if (!(flags & _DPRINTK_FLAGS_PRINT))
+                                        static_branch_disable(&dp->key.dd_key_true);
+                        } else if (flags & _DPRINTK_FLAGS_PRINT)
+                                static_branch_enable(&dp->key.dd_key_true);
+#endif
                         dp->flags = newflags;
                         vpr_info("changed %s:%d [%s]%s =%s\n",
                                  trim_prefix(dp->filename), dp->lineno,