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authorSteven Rostedt (VMware) <rostedt@goodmis.org>2017-01-17 12:29:35 -0500
committerSteven Rostedt (VMware) <rostedt@goodmis.org>2017-01-17 15:13:05 -0500
commitd45ae1f7041ac52ade6c5ec76d96bbed765d67aa (patch)
tree902c94fd3caee27e8812eebb0591d10243fcb871 /kernel/trace/trace_branch.c
parent6496bb72bf20c1c7e4d6be44dfa663163e709116 (diff)
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>
Diffstat (limited to 'kernel/trace/trace_branch.c')
-rw-r--r--kernel/trace/trace_branch.c6
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