s390/ftrace: optimize function graph caller code

When the function graph tracer is disabled we can skip three additional
instructions. So let's just do this.

So if function tracing is enabled but function graph tracing is
runtime disabled, we get away with a single unconditional branch.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

3 files changed, 34 insertions, 6 deletions

diff --git a/arch/s390/include/asm/ftrace.h b/arch/s390/include/asm/ftrace.h
index bf246dae1367..7b8e456d76c9 100644
--- a/arch/s390/include/asm/ftrace.h
+++ b/arch/s390/include/asm/ftrace.h
@@ -4,6 +4,7 @@
 #ifndef __ASSEMBLY__
 
 extern void _mcount(void);
+extern char ftrace_graph_caller_end;
 
 struct dyn_arch_ftrace { };
 
diff --git a/arch/s390/kernel/ftrace.c b/arch/s390/kernel/ftrace.c
index 54d6493c4a56..de55efa5b64e 100644
--- a/arch/s390/kernel/ftrace.c
+++ b/arch/s390/kernel/ftrace.c
@@ -170,6 +170,29 @@ out:
  * directly after the instructions. To enable the call we calculate
  * the original offset to prepare_ftrace_return and put it back.
  */
+
+#ifdef CONFIG_64BIT
+
+int ftrace_enable_ftrace_graph_caller(void)
+{
+        static unsigned short offset = 0x0002;
+
+        return probe_kernel_write((void *) ftrace_graph_caller + 2,
+                                  &offset, sizeof(offset));
+}
+
+int ftrace_disable_ftrace_graph_caller(void)
+{
+        unsigned short offset;
+
+        offset = ((void *) &ftrace_graph_caller_end -
+                  (void *) ftrace_graph_caller) / 2;
+        return probe_kernel_write((void *) ftrace_graph_caller + 2,
+                                  &offset, sizeof(offset));
+}
+
+#else /* CONFIG_64BIT */
+
 int ftrace_enable_ftrace_graph_caller(void)
 {
         unsigned short offset;
@@ -188,5 +211,6 @@ int ftrace_disable_ftrace_graph_caller(void)
                                   &offset, sizeof(offset));
 }
 
+#endif /* CONFIG_64BIT */
 #endif /* CONFIG_DYNAMIC_FTRACE */
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
diff --git a/arch/s390/kernel/mcount64.S b/arch/s390/kernel/mcount64.S
index c67a8bf0fd9a..5b33c83adde9 100644
--- a/arch/s390/kernel/mcount64.S
+++ b/arch/s390/kernel/mcount64.S
@@ -32,14 +32,17 @@ ENTRY(ftrace_caller)
         lg      %r14,0(%r14)
         basr    %r14,%r14
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
+# The j instruction gets runtime patched to a nop instruction.
+# See ftrace_enable_ftrace_graph_caller. The patched instruction is:
+#       j       .+4
+ENTRY(ftrace_graph_caller)
+        j       ftrace_graph_caller_end
         lg      %r2,168(%r15)
         lg      %r3,272(%r15)
-ENTRY(ftrace_graph_caller)
-# The bras instruction gets runtime patched to call prepare_ftrace_return.
-# See ftrace_enable_ftrace_graph_caller. The patched instruction is:
-#       bras    %r14,prepare_ftrace_return
-        bras    %r14,0f
-0:      stg     %r2,168(%r15)
+        brasl   %r14,prepare_ftrace_return
+        stg     %r2,168(%r15)
+ftrace_graph_caller_end:
+        .globl  ftrace_graph_caller_end
 #endif
         aghi    %r15,160
         lmg     %r2,%r5,32(%r15)