Re: [BUG] perf report: ordered events and flushing bug

Adrian Hunter
2015-03-12T09:04:48Z

On 12/03/15 05:32, Stephane Eranian wrote:
> Hi,
>
> I am working on the JIT support to improve the flow and have
> perf record inject the MMAPs at the end of the collection. For
> that I piggyback on the buildid pass. To avoid rewriting the entire perf.data
> file, I simply append the the MMAP records at the end of the file. And that
> puts them out-of-order in time systematically. But I thought it would be okay
> because perf report would sort them by timestamps again.
>
> Well, it does not in all cases! Why?
>
> Because of the round flushing. Based on how far out-of-order an event is,
> it may not be processed correctly because of round flushing. I believe this
> may only impact MMAP records. This is a serious issue because
> mmaps drive symbolization of samples. If samples are processed without the
> proper dso mapping, then samples may not be symbolized or may be wrongly
> symbolized.
>
> So far, the workaround I found was to set the oe->next_flush = 0 for
> the ROUND mode.
> In other words, do not flush anything until FINAL. To me, this is the
> only sensible
> way of avoiding this kind of problems. I am not sure I understand the
> point of flushing
> anyway, except to minimize memory footprint, maybe. But it does not
> work with vastly
> out-of-order mmaps.
>
> Do you have a better solution?

You could hook the ordered event delivery (see Arnaldo's perf/core branch):

if (tool->ordered_events) {
inject->deliver = session->ordered_events.deliver;
session->ordered_events.deliver = jit_mmap_deliver;
}

int jit_mmap_deliver(struct ordered_events *oe, struct ordered_event *event, struct perf_sample *sample)
{
struct perf_inject *inject = container_of(oe->tool, struct perf_inject, tool);

if (next_jit_mmap_time < sample->time) {
<synth mmap event>
perf_session__deliver_synth_event(...);
}
return inject->deliver(oe, event, sample);
}

Need to get Arnaldo's comment on this approach first though.