perf tools: Document --children option in more detail

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 ;-).

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Taeung Song <treeze.taeung@gmail.com>
Link: http://lkml.kernel.org/r/1429684425-14987-1-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
This commit is contained in:
Namhyung Kim 2015-04-22 15:33:45 +09:00 committed by Arnaldo Carvalho de Melo
parent c4fa0d9c1e
commit dd3092075c
3 changed files with 114 additions and 1 deletions

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@ -0,0 +1,108 @@
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; there are three functions like below.
-----------------------
void foo(void) {
/* do 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 show 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 values of
child functions (i.e. 'foo' and 'bar') are added to the parents 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
-------------------------------------------
In the above output, the 'self' overhead of 'foo' (60%) was add to the
'children' overhead of 'bar', 'main' and '\_\_libc_start_main'.
Likewise, the 'self' overhead of 'bar' (40%) was added to the
'children' overhead of 'main' and '\_\_libc_start_main'.
So '\_\_libc_start_main' and 'main' are shown first since they have
same (100%) 'children' overhead (even though they have zero 'self'
overhead) and they are the parents of 'foo' and 'bar'.
Since v3.16 the 'children' overhead is shown by default and the output
is sorted by its values. The '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.

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@ -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::callchain-overhead-calculation.txt[]
SEE ALSO
--------
linkperf:perf-stat[1], linkperf:perf-annotate[1]

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@ -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::callchain-overhead-calculation.txt[]
SEE ALSO
--------