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

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Date:	Thu, 12 Mar 2015 15:05:42 -0400
From:	Stephane Eranian <eranian@...gle.com>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Jiri Olsa <jolsa@...hat.com>, David Ahern <dsahern@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...e.hu" <mingo@...e.hu>
Subject: Re: [BUG] perf report: ordered events and flushing bug

On Thu, Mar 12, 2015 at 5:02 AM, Adrian Hunter <adrian.hunter@...el.com> wrote:
> 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);
> }
>
I don't quite understand what this does?
Are you saying that in perf/core there is a new callback mechanism to
override the deliver phase
when you need ordered event and that callback could be used to avoid
the flushing?
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