[for-next][PATCH 2/4] tracing: Process constants for (un)likely() profiler

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>

When running the likely/unlikely profiler, one of the results did not look
accurate. It noted that the unlikely() in link_path_walk() was 100%
incorrect. When I added a trace_printk() to see what was happening there, it
became 80% correct! Looking deeper into what whas happening, I found that
gcc split that if statement into two paths. One where the if statement
became a constant, the other path a variable. The other path had the if
statement always hit (making the unlikely there, always false), but since
the #define unlikely() has:

  #define unlikely() (__builtin_constant_p(x) ? !!(x) : __branch_check__(x, 0))

Where constants are ignored by the branch profiler, the "constant" path
made by the compiler was ignored, even though it was hit 80% of the time.

By just passing the constant value to the __branch_check__() function and
tracing it out of line (as always correct, as likely/unlikely isn't a factor
for constants), then we get back the accurate readings of branches that were
optimized by gcc causing part of the execution to become constant.

Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
 include/linux/compiler.h    | 14 ++++++++------
 kernel/trace/trace_branch.c |  6 +++++-
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index cf0fa5d86059..bbbe1570de1c 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -107,12 +107,13 @@ struct ftrace_branch_data {
  */
 #if defined(CONFIG_TRACE_BRANCH_PROFILING) \
     && !defined(DISABLE_BRANCH_PROFILING) && !defined(__CHECKER__)
-void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
+void ftrace_likely_update(struct ftrace_branch_data *f, int val,
+			  int expect, int is_constant);
 
 #define likely_notrace(x)	__builtin_expect(!!(x), 1)
 #define unlikely_notrace(x)	__builtin_expect(!!(x), 0)
 
-#define __branch_check__(x, expect) ({					\
+#define __branch_check__(x, expect, is_constant) ({			\
 			int ______r;					\
 			static struct ftrace_branch_data		\
 				__attribute__((__aligned__(4)))		\
@@ -122,8 +123,9 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
 				.file = __FILE__,			\
 				.line = __LINE__,			\
 			};						\
-			______r = likely_notrace(x);			\
-			ftrace_likely_update(&______f, ______r, expect); \
+			______r = __builtin_expect(!!(x), expect);	\
+			ftrace_likely_update(&______f, ______r,		\
+					     expect, is_constant);	\
 			______r;					\
 		})
 
@@ -133,10 +135,10 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
  * written by Daniel Walker.
  */
 # ifndef likely
-#  define likely(x)	(__builtin_constant_p(x) ? !!(x) : __branch_check__(x, 1))
+#  define likely(x)	(__branch_check__(x, 1, __builtin_constant_p(x)))
 # endif
 # ifndef unlikely
-#  define unlikely(x)	(__builtin_constant_p(x) ? !!(x) : __branch_check__(x, 0))
+#  define unlikely(x)	(__branch_check__(x, 0, __builtin_constant_p(x)))
 # endif
 
 #ifdef CONFIG_PROFILE_ALL_BRANCHES
diff --git a/kernel/trace/trace_branch.c b/kernel/trace/trace_branch.c
index 75489de546b6..7afe426ea528 100644
--- a/kernel/trace/trace_branch.c
+++ b/kernel/trace/trace_branch.c
@@ -200,8 +200,12 @@ void trace_likely_condition(struct ftrace_branch_data *f, int val, int expect)
 }
 #endif /* CONFIG_BRANCH_TRACER */
 
-void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect)
+void ftrace_likely_update(struct ftrace_branch_data *f, int val,
+			  int expect, int is_constant)
 {
+	/* A constant is always correct */
+	if (is_constant)
+		val = expect;
 	/*
 	 * I would love to have a trace point here instead, but the
 	 * trace point code is so inundated with unlikely and likely
-- 
2.10.2