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SubjectRe: [PATCH v5 03/17] perf ftrace: add option -t/--tid to filter by thread id
On Fri, 17 Jul 2020 16:34:55 -0300
Arnaldo Carvalho de Melo <acme@kernel.org> wrote:

Thinking a bit more, I have to ask. Does perf use the kernel when
getting all the children of an existing task, or is that done only in
userspace?

That is, is there a perf syscall that says "start tracing this task and
all its existing children"?

Or is it done by perf user space looking at the /proc filesystem (like
ps does).

I'm asking because if perf has a syscall to do that, then I probably
should add a way to do that with ftrace as well. But that's really
trivial, because all it would take is grabbing the task_list lock and
iterating over all the children. Getting new children was the
non-trivial part, which was what I focused on (with the fork options).

If perf does it with proc files, then we don't need to change anything
as that could still be used with ftrace.

> Changbin, you can take from here :-)
>
> And to reiterate, for me the value of 'perf ftrace' is to allow people
> used to perf to be able to switch to ftrace quickly, just changing:
>
> perf record/top/stat/trace/report/script/etc --pid 1234
>
> by:
>
> perf ftrace --pid 1234
>
> And have the tracefs ftrace knobs set up to have what is expected in
> terms of targets to trace as the other perf tools.
>
> And not just --pid and --tid, but --cgroup, --cpu, etc.
>
> i.e., 'perf ftrace' being _a_ front-end aplication to ftrace.
>
> :-)


I have no problem with this, and I'm quite excited about it. I would
like it to use libtracefs, as it looks to be exactly what we are
working on. And this is now a high priority to get out, and I don't
expect another year (or two) in doing so ;-)

-- Steve

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