perf ftrace: Select function/function_graph tracer automatically

The '-g/-G' options have already implied function_graph tracer should be
used instead of function tracer. So we don't need extra option
'--tracer' in this case.

This patch changes the behavior as below:

  - If '-g' or '-G' option is on, then function_graph tracer is used.
  - If '-T' or '-N' option is on, then function tracer is used.
  - The function_graph has priority over function tracer.
  - The option '--tracer' only take effect if neither -g/-G nor -T/-N
    is specified.

Here are some examples.

This will start tracing all functions using default tracer:

  $ sudo perf ftrace

This will trace all functions using function graph tracer:

  $ sudo perf ftrace -G '*'

This will trace function vfs_read using function graph tracer:

  $ sudo perf ftrace -G vfs_read

This will trace function vfs_read using function tracer:

  $ sudo perf ftrace -T vfs_read

Committer notes:

Using '-h -G' will tell what that option is about, so to further clarify
the above examples:

  # perf ftrace -h -G

    -G, --graph-funcs <func>	Set graph filter on given functions

  # perf ftrace -h -g

    -g, --nograph-funcs <func>	Set nograph filter on given functions

  # perf ftrace -h -T

    -T, --trace-funcs <func>	trace given functions only

  # perf ftrace -h -N

    -N, --notrace-funcs <func>	do not trace given functions

  #

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: http://lore.kernel.org/lkml/20200808023141.14227-2-changbin.du@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

2 files changed, 25 insertions, 21 deletions

diff --git a/tools/perf/Documentation/perf-config.txt b/tools/perf/Documentation/perf-config.txt
index c7d3df5798e2..76408d986aed 100644
--- a/tools/perf/Documentation/perf-config.txt
+++ b/tools/perf/Documentation/perf-config.txt
@@ -614,8 +614,9 @@ trace.*::
 
 ftrace.*::
 	ftrace.tracer::
-		Can be used to select the default tracer. Possible values are
-		'function' and 'function_graph'.
+		Can be used to select the default tracer when neither -G nor
+		-F option is not specified. Possible values are 'function' and
+		'function_graph'.
 
 llvm.*::
 	llvm.clang-path::
diff --git a/tools/perf/Documentation/perf-ftrace.txt b/tools/perf/Documentation/perf-ftrace.txt
index b80c84307dc9..821d4d334a09 100644
--- a/tools/perf/Documentation/perf-ftrace.txt
+++ b/tools/perf/Documentation/perf-ftrace.txt
@@ -24,7 +24,8 @@ OPTIONS
 
 -t::
 --tracer=::
-	Tracer to use: function_graph or function.
+	Tracer to use when neither -G nor -F option is not
+	specified: function_graph or function.
 
 -v::
 --verbose=::
@@ -50,33 +51,35 @@ OPTIONS
 
 -T::
 --trace-funcs=::
-	Only trace functions given by the argument.  Multiple functions
-	can be given by using this option more than once.  The function
-	argument also can be a glob pattern.  It will be passed to
-	'set_ftrace_filter' in tracefs.
+	Select function tracer and set function filter on the given
+	function (or a glob pattern). Multiple functions can be given
+	by using this option more than once. The function argument also
+	can be a glob pattern. It will be passed to 'set_ftrace_filter'
+	in tracefs.
 
 -N::
 --notrace-funcs=::
-	Do not trace functions given by the argument.  Like -T option,
-	this can be used more than once to specify multiple functions
-	(or glob patterns).  It will be passed to 'set_ftrace_notrace'
-	in tracefs.
+	Select function tracer and do not trace functions given by the
+	argument.  Like -T option, this can be used more than once to
+	specify multiple functions (or glob patterns).  It will be
+	passed to 'set_ftrace_notrace' in tracefs.
 
 -G::
 --graph-funcs=::
-	Set graph filter on the given function (or a glob pattern).
-	This is useful for the function_graph tracer only and enables
-	tracing for functions executed from the given function.
-	This can be used more than once to specify multiple functions.
-	It will be passed to 'set_graph_function' in tracefs.
+	Select function_graph tracer and set graph filter on the given
+	function (or a glob pattern). This is useful to trace for
+	functions executed from the given function. This can be used more
+	than once to specify multiple functions. It will be passed to
+	'set_graph_function' in tracefs.
 
 -g::
 --nograph-funcs=::
-	Set graph notrace filter on the given function (or a glob pattern).
-	Like -G option, this is useful for the function_graph tracer only
-	and disables tracing for function executed from the given function.
-	This can be used more than once to specify multiple functions.
-	It will be passed to 'set_graph_notrace' in tracefs.
+	Select function_graph tracer and set graph notrace filter on the
+	given function (or a glob pattern). Like -G option, this is useful
+	for the function_graph tracer only and disables tracing for function
+	executed from the given function. This can be used more than once to
+	specify multiple functions. It will be passed to 'set_graph_notrace'
+	in tracefs.
 
 -D::
 --graph-depth=::