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    SubjectRe: [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@goodmis.org> wrote:
    >
    > 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).

    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

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