linux-kernel - Re: [PATCH v5 03/17] perf ftrace: add option -t/--tid to filter by thread id

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Date:   Tue, 21 Jul 2020 22:44:44 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Changbin Du <changbin.du@...il.com>,
        Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 03/17] perf ftrace: add option -t/--tid to filter by
 thread id

Hi Steve,

On Sat, Jul 18, 2020 at 4:44 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Fri, 17 Jul 2020 16:34:55 -0300
> Arnaldo Carvalho de Melo <acme@...nel.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).

Yep, perf does look up the /proc to get a list of threads in a process.

Thanks
Namhyung


>
> 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