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    SubjectRe: [RFC PATCH 0/2] perf_events: add support for per-cpu per-cgroup monitoring (v3)
    On Tue, Sep 21, 2010 at 6:27 PM, Peter Zijlstra <> wrote:
    > On Tue, 2010-09-21 at 18:17 +0200, Stephane Eranian wrote:
    >> On Tue, Sep 21, 2010 at 4:03 PM, Peter Zijlstra <> wrote:
    >> > On Tue, 2010-09-21 at 15:38 +0200, Stephane Eranian wrote:
    >> >> > Hmm, indeed. One thing we can do about that is move perf into the
    >> >> > cgroup, create the counter (disabled) using self to identify the cgroup,
    >> >> > move perf back to where it came from, and enable the counter.
    >> >> >
    >> >> Yes, that's another possibility. I wonder if there are any non-obvious
    >> >> difficulties with this approach.
    >> >
    >> > Yes, there is, but I think we can fix it. The problem with moving perf
    >> > itself around is that perf is not a fully dormant process and can thus
    >> > interact with the cgroup state.
    >> >
    >> I was thinking about memory accounting for instance.
    > I think the memory controller only accounts things when the process
    > actually touches something. A process that never wakes will never touch
    > anything.
    >> > If we were to fork a child that's simply sitting idle in waitpid() (or
    >> > any other blocking syscall) we can move that around cgroup without
    >> > affecting the cgroup itself.
    >> But then things get a bit more complicated because the perf_event_open()
    >> has to be done in that child. File descriptors created in child processes
    >> and not shared with their parent. You'd have to pass file descriptors around.
    >> That seems overly complicated.
    > Uhm, no the trick is that the child remains absolutely dormant and
    > therefore doesn't accrue any accounting, all you need is a known task in
    > the cgroup, the parent can then specify the child pid to identify the
    > group.
    > Once you've opened the counter, you can move the kid out and kill it.
    > Note that moving it out of the cgroup before killing it ensure it never
    > wakes up inside that cgroup.
    Ok, I see. I got confused by the 'self' pid. The parent moves the dormant
    child into the cgroup and uses its pid in perf_event_open(). The existing
    logic inside the tool remains unmodified.

    I will experiment with this approach.

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