[PATCH v3] perf tools: Document --children option in more detail

[Namhyung Kim]

As the --children option changes the output of perf report (and perf
top) it sometimes confuses users.  Add more words and examples to help
understanding of the option's behavior - and how to disable it ;-).

Reviewed-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Taeung Song <treeze.taeung@xxxxxxxxx>
Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
---
 tools/perf/Documentation/overhead-calculation.txt | 97 +++++++++++++++++++++++
 tools/perf/Documentation/perf-report.txt          |  4 +
 tools/perf/Documentation/perf-top.txt             |  3 +-
 3 files changed, 103 insertions(+), 1 deletion(-)
 create mode 100644 tools/perf/Documentation/overhead-calculation.txt

diff --git a/tools/perf/Documentation/overhead-calculation.txt b/tools/perf/Documentation/overhead-calculation.txt
new file mode 100644
index 000000000000..d7095f77154c
--- /dev/null
+++ b/tools/perf/Documentation/overhead-calculation.txt
@@ -0,0 +1,97 @@
+Overhead calculation
+--------------------
+The overhead can be shown in two columns as 'Children' and 'Self' when
+perf collects callchains.  The self overhead is simply calculated by
+adding all period values of the entry - usually a function (symbol).
+This is the value that perf shows traditionally and sum of all the
+self overhead values should be 100%.
+
+The 'children' overhead is calculated by adding all period values of
+the child functions so that it can show the total overhead of the
+higher level functions even if they don't directly execute much.
+'Children' here means functions that are called from another (parent)
+function.
+
+It might be confusing that the sum of all the 'children' overhead
+values exceeds 100% since each of them is already an accumulation of
+'self' overhead of its child functions.  But with this enabled, users
+can find which function has the most overhead even if samples are
+spread over the children.
+
+Consider the following example; thereâre three functions like below.
+
+-----------------------
+void foo(void) {
+    /* something */
+}
+
+void bar(void) {
+    /* do something */
+    foo();
+}
+
+int main(void) {
+    bar()
+    return 0;
+}
+-----------------------
+
+In this case 'foo' is a child of 'bar', and 'bar' is an immediate
+child of 'main' so 'foo' also is a child of 'main'.  In other words,
+'main' is a parent of 'foo' and 'bar', and 'bar' is a parent of 'foo'.
+
+Suppose all samples are recorded in 'foo' and 'bar' only.  When it's
+recorded with callchains the output will be shown something like below
+in the usual (self-overhead-only) output of perf report:
+
+----------------------------------
+Overhead  Symbol
+........  .....................
+  60.00%  foo
+          |
+          --- foo
+              bar
+              main
+              __libc_start_main
+
+  40.00%  bar
+          |
+          --- bar
+              main
+              __libc_start_main
+----------------------------------
+
+When the --children option is enabled, the (self) overhead of children (in
+this case foo and bar) are added to the parent to calculate the
+children overhead.  In this case the report could be displayed as:
+
+-------------------------------------------
+Children      Self  Symbol
+........  ........  ....................
+ 100.00%     0.00%  __libc_start_main
+          |
+          --- __libc_start_main
+
+ 100.00%     0.00%  main
+          |
+          --- main
+              __libc_start_main
+
+ 100.00%    40.00%  bar
+          |
+          --- bar
+              main
+              __libc_start_main
+
+  60.00%    60.00%  foo
+          |
+          --- foo
+              bar
+              main
+              __libc_start_main
+-------------------------------------------
+
+Since v3.16 the children overhead is shown by default and the output
+is sorted by the values. Children overhead is disabled by specifying
+--no-children option on the command line or by adding 'report.children
+= false' or 'top.children = false' in the perf config file.
diff --git a/tools/perf/Documentation/perf-report.txt b/tools/perf/Documentation/perf-report.txt
index 4879cf638824..b7bb298deee3 100644
--- a/tools/perf/Documentation/perf-report.txt
+++ b/tools/perf/Documentation/perf-report.txt
@@ -193,6 +193,7 @@ OPTIONS
 	Accumulate callchain of children to parent entry so that then can
 	show up in the output.  The output will have a new "Children" column
 	and will be sorted on the data.  It requires callchains are recorded.
+	See the `overhead calculation' section for more details.
 
 --max-stack::
 	Set the stack depth limit when parsing the callchain, anything
@@ -323,6 +324,9 @@ OPTIONS
 --header-only::
 	Show only perf.data header (forces --stdio).
 
+
+include::overhead-calculation.txt[]
+
 SEE ALSO
 --------
 linkperf:perf-stat[1], linkperf:perf-annotate[1]
diff --git a/tools/perf/Documentation/perf-top.txt b/tools/perf/Documentation/perf-top.txt
index 3265b1070518..6e8fea6fd179 100644
--- a/tools/perf/Documentation/perf-top.txt
+++ b/tools/perf/Documentation/perf-top.txt
@@ -168,7 +168,7 @@ Default is to monitor all CPUS.
 	Accumulate callchain of children to parent entry so that then can
 	show up in the output.  The output will have a new "Children" column
 	and will be sorted on the data.  It requires -g/--call-graph option
-	enabled.
+	enabled.  See the `overhead calculation' section for more details.
 
 --max-stack::
 	Set the stack depth limit when parsing the callchain, anything
@@ -234,6 +234,7 @@ INTERACTIVE PROMPTING KEYS
 
 Pressing any unmapped key displays a menu, and prompts for input.
 
+include::overhead-calculation.txt[]
 
 SEE ALSO
 --------
-- 
2.3.5

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