tracing: likely/unlikely branch annotation tracer

Impact: new likely/unlikely branch tracer

This patch adds a way to record the instances of the likely() and unlikely()
branch condition annotations.

When "unlikely" is set in /debugfs/tracing/iter_ctrl the unlikely conditions
will be added to any of the ftrace tracers. The change takes effect when
a new tracer is passed into the current_tracer file.

For example:

 bash-3471  [003]   357.014755: [INCORRECT] sched_info_dequeued:sched_stats.h:177
 bash-3471  [003]   357.014756: [correct] update_curr:sched_fair.c:489
 bash-3471  [003]   357.014758: [correct] calc_delta_fair:sched_fair.c:411
 bash-3471  [003]   357.014759: [correct] account_group_exec_runtime:sched_stats.h:356
 bash-3471  [003]   357.014761: [correct] update_curr:sched_fair.c:489
 bash-3471  [003]   357.014763: [INCORRECT] calc_delta_fair:sched_fair.c:411
 bash-3471  [003]   357.014765: [correct] calc_delta_mine:sched.c:1279

Which shows the normal tracer heading, as well as whether the condition was
correct "[correct]" or was mistaken "[INCORRECT]", followed by the function,
file name and line number.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 22 insertions, 0 deletions

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index a604f24c755f..8abcaf821beb 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -175,6 +175,28 @@ config TRACE_UNLIKELY_PROFILE
 
           Say N if unsure.
 
+config TRACING_UNLIKELY
+        bool
+        help
+          Selected by tracers that will trace the likely and unlikely
+          conditions. This prevents the tracers themselves from being
+          profiled. Profiling the tracing infrastructure can only happen
+          when the likelys and unlikelys are not being traced.
+
+config UNLIKELY_TRACER
+        bool "Trace likely/unlikely instances"
+        depends on TRACE_UNLIKELY_PROFILE
+        select TRACING_UNLIKELY
+        help
+          This traces the events of likely and unlikely condition
+          calls in the kernel.  The difference between this and the
+          "Trace likely/unlikely profiler" is that this is not a
+          histogram of the callers, but actually places the calling
+          events into a running trace buffer to see when and where the
+          events happened, as well as their results.
+
+          Say N if unsure.
+
 config STACK_TRACER
         bool "Trace max stack"
         depends on HAVE_FUNCTION_TRACER