tracing: Process constants for (un)likely() profiler

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@goodmis.org>

1 files changed, 5 insertions, 1 deletions

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