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    SubjectRe: [PATCH 35/40] perf record: Synthesize COMM event for a command line workload
    On Tue, May 19, 2015 at 11:02:20AM -0300, Arnaldo Carvalho de Melo wrote:
    > Em Tue, May 19, 2015 at 04:46:43PM +0900, Namhyung Kim escreveu:
    > > On Mon, May 18, 2015 at 09:45:35AM -0300, Arnaldo Carvalho de Melo wrote:
    > > > Em Mon, May 18, 2015 at 09:30:50AM +0900, Namhyung Kim escreveu:
    > > > > When perf creates a new child to profile, the events are enabled on
    > > > > exec(). And in this case, it doesn't synthesize any event for the
    > > > > child since they'll be generated during exec(). But there's an window
    > > > > between the enabling and the event generation.
    > > > >
    > > > > It used to be overcome since samples are only in kernel (so we always
    > > > > have the map) and the comm is overridden by a later COMM event.
    > > > > However it won't work anymore since those samples will go to a missing
    > > > > thread now but the COMM event will create a (current) thread. This
    > > > > leads to those early samples (like native_write_msr_safe) not having a
    > > > > comm but pid (like ':15328').
    > > >
    > > > > So it needs to synthesize COMM event for the child explicitly before
    > > > > enabling so that it can have a correct comm. But at this time, the
    > > > > comm will be "perf" since it's not exec-ed yet.
    > > >
    > > > This looks reasonable, but I think it probably needs to be done
    > > > somewhere in perf_evlist__prepare_workload() or
    > > > perf_evlist__start_workload(), as this affects other tools as well, like
    > > > 'top', 'trace' and any other that may want to do this start-workload use
    > > > case.
    > >
    > > Hmm.. I need to look at this again as it only affects on processing
    > > indexed data files which used to have a separate missing threads tree.
    >
    > Humm, you're thinking about where you managed to reproduce the problem,
    > I am thinking outside indexing, etc, i.e. by definition we either enable
    > the event before we fork, so that we get the PERF_RECORD_FORK/COMM or we
    > synthesize it either from /proc or directly (preferred) if we decide to
    > do it after the fork/exec, right?

    But as I said before, later COMM event will override thread->comm to a
    proper string as long as it can find a matching thread. So I think it
    has no problem in the current code.

    In the old version of this patchset (v3), indexing made it impossible
    for COMM event to find a matching thread since it used to have a
    separate tree for threads that have sampled before any FORK/COMM event
    came. I think it doesn't apply to the current version anymore, I
    will check it tomorrow.

    Thanks,
    Namhyung


    >
    > - Arnaldo
    >
    > > That's the reason why I didn't put it in a generic place like you
    > > said.
    > >
    > > However I changed not to use the separate tree - the purpose of the
    > > tree was to reduce lock acquisition on thread searching but it already
    > > grabs a rwlock with thread refcounting change.
    > >
    > > Will check whether this is still needed..
    > >
    > > Thanks,
    > > Namhyung
    > >
    > >
    > > >
    > > > I also wonder if we can't overcome this without using /proc, i.e.
    > > > actually moving the "start the workload" to just before the fork, so
    > > > that the kernel covers that as well.
    > > >
    > > > Or, alternatively, the thread can be created without having to look at
    > > > /proc at all, but by directly creating the struct thread, with the
    > > > correct COMM, pid, etc, that we know, since we forked it, etc.


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