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

Namhyung Kim
2015-03-12T08:03:24Z

Hi Stephane,

On Wed, Mar 11, 2015 at 11:32:15PM -0400, 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 may want to see my multi-thread work [1] for this. It supports
out-of-time processing of samples by splitting metadata and samples
with index. I think you can just inject the MMAPs to the metadata
area - yes, it requires whole file rewrite anyway but you can skip the
sample data parts.

[1] https://lkml.org/lkml/2015/3/2/988

Thanks,
Namhyung