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SubjectRe: [PATCH v2] perf tools: Document --children option in more detail

* Namhyung Kim <namhyung@kernel.org> wrote:

> 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 ;-).

Nice! :-)

> +++ b/tools/perf/Documentation/overhead.txt

As some people might stumble upon this the old fashioned way, by
looking around in 'Documentation/', I'm wondering how this file name
will tell the reader that this is about call chains? (which isn't the
default recording mode) Maybe name it more explicitly, like
callchain-overhead.txt?

> @@ -0,0 +1,96 @@
> +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 the all
> +self overhead should be 100%.

s/sum of the all self overhead should be 100%
/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. The children here means that functions called from
> +another function.

Maybe put it into quotes to specify it more clearly? Also, maybe add a
bit more info - Something like:

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 the all children overhead

s/children
/'children'

> +exceeds 100% since each of them is already an accumulation of (self)
> +overhead of its children. 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; there’re three functions like below.
> +
> +-----------------------
> +void foo(void) {
> + /* something */
> +}
> +
> +bar(void) {

s/bar(void) {
/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'.

s/and 'bar'. and
/and 'bar', and

> +
> +Suppose all samples are recorded in the 'foo' and 'bar' only. When

either:
Suppose all samples are recorded in the 'foo' and 'bar' functions only.

or:
Suppose all samples are recorded in 'foo' and 'bar' only.

> +you record with callchain you'll see something like below in the usual

s/callchain/callchains

> +(self-overhead-only) output of the perf report:

s/of the perf report/of perf report

> +
> +----------------------------------
> +Overhead Symbol
> +........ .....................
> + 60.00% foo
> + |
> + --- foo
> + bar
> + main
> + __libc_start_main
> +
> + 40.00% bar
> + |
> + --- bar
> + main
> + __libc_start_main
> +----------------------------------
> +
> +When --children option is enabled, the (self) overhead of children (in

s/When --children option
/When the --children option

> +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 perfconfig file.

s/perfconfig/.perfconfig

?


> diff --git a/tools/perf/Documentation/perf-report.txt b/tools/perf/Documentation/perf-report.txt
> index 4879cf638824..bd97e5c3c9b6 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 `overhead calculation' section for more details.

s/See `overhead calculation' section
/See the `overhead calculation' section

>
> --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.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..526d6bebec1f 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 `overhead calculation' section for more details.

s/See `overhead calculation' section
/See the `overhead calculation' section

With the above things fixed:

Reviewed-by: Ingo Molnar <mingo@kernel.org>

Thanks,

Ingo

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