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SubjectRe: [PATCH] perf tools: update docs regarding kernel/user space unwinding
Em Wed, Mar 25, 2020 at 09:40:53AM -0700, Tony Jones escreveu:
> The method of unwinding for kernel space is defined by the kernel config,
> not by the value of --call-graph. Improve the documentation to reflect
> this.

Fixed the callgraph -> call-graph bit, as you pointed out privately,
applied.

About your question, to get the answer in some public location as
documentation about perf usage:

> As an aside, for record path, do you know where PERF_SAMPLE_CALLCHAIN
> is actually set before being passed to kernel space?

So, I think is somewhere down from perf_evsel__config()... its in:

perf_evsel__set_sample_bit(evsel, CALLCHAIN);

which is set at:

$ perf probe -x ~/bin/perf -L __perf_evsel__config_callchain
<__perf_evsel__config_callchain@/home/acme/git/perf/tools/perf/util/evsel.c:0>
0 static void __perf_evsel__config_callchain(struct evsel *evsel,
struct record_opts *opts,
struct callchain_param *param)
3 {
4 bool function = perf_evsel__is_function_event(evsel);
5 struct perf_event_attr *attr = &evsel->core.attr;

7 perf_evsel__set_sample_bit(evsel, CALLCHAIN);

9 attr->sample_max_stack = param->max_stack;

11 if (opts->kernel_callchains)
12 attr->exclude_callchain_user = 1;
13 if (opts->user_callchains)
14 attr->exclude_callchain_kernel = 1;
15 if (param->record_mode == CALLCHAIN_LBR) {


Line 7 of __perf_evsel__config_callchain(), so lets use perf probe +
perf trace + perf callchains to see where perf callchains are asked from
the kernel:

[root@seventh ~]# perf probe -x ~/bin/perf __perf_evsel__config_callchain:7
Added new event:
probe_perf:__perf_evsel__config_callchain_L7 (on __perf_evsel__config_callchain:7 in /home/acme/bin/perf)

You can now use it in all perf tools, such as:

perf record -e probe_perf:__perf_evsel__config_callchain_L7 -aR sleep 1

[root@seventh ~]#
[root@seventh ~]# perf trace -e probe_perf:*callchain*/max-stack=16/ perf record -g sleep 1
0.000 perf/14860 probe_perf:__perf_evsel__config_callchain_L7(__probe_ip: 5263069)
__perf_evsel__config_callchain (/home/acme/bin/perf)
perf_evsel__config_callchain (/home/acme/bin/perf)
perf_evsel__config (/home/acme/bin/perf)
perf_evlist__config (/home/acme/bin/perf)
record__open (/home/acme/bin/perf)
__cmd_record (/home/acme/bin/perf)
cmd_record (/home/acme/bin/perf)
run_builtin (/home/acme/bin/perf)
handle_internal_command (/home/acme/bin/perf)
run_argv (/home/acme/bin/perf)
main (/home/acme/bin/perf)
__libc_start_main (/usr/lib64/libc-2.29.so)
[0] ([unknown])
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.020 MB perf.data (8 samples) ]
[root@seventh ~]#

That [0] is the ugly part here, have seen it before, need to nail it
down, unsee it and all the rest seems ok, right?

- Arnaldo

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