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author | Steven Rostedt (VMware) <rostedt@goodmis.org> | 2017-01-17 12:29:35 -0500 |
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committer | Steven Rostedt (VMware) <rostedt@goodmis.org> | 2017-01-17 15:13:05 -0500 |
commit | d45ae1f7041ac52ade6c5ec76d96bbed765d67aa (patch) | |
tree | 902c94fd3caee27e8812eebb0591d10243fcb871 /kernel/trace/trace_branch.c | |
parent | 6496bb72bf20c1c7e4d6be44dfa663163e709116 (diff) | |
download | linux-d45ae1f7041ac52ade6c5ec76d96bbed765d67aa.tar.bz2 |
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.c | 6 |
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 |