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>
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index cf0fa5d..bbbe157 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -107,12 +107,13 @@
*/
#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 @@
.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 @@
* 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