Re: [PATCH v3 07/12] perf record: init data file at mmap buffer object

[Namhyung Kim]

On Mon, Mar 1, 2021 at 10:33 PM Bayduraev, Alexey V
<alexey.v.bayduraev@xxxxxxxxxxxxxxx> wrote:
>
> On 01.03.2021 14:44, Namhyung Kim wrote:
> > Hello,
> >
> > On Mon, Mar 1, 2021 at 8:16 PM Bayduraev, Alexey V
> > <alexey.v.bayduraev@xxxxxxxxxxxxxxx> wrote:
> >>
> >> Hi,
> >>
> >> On 20.11.2020 13:49, Namhyung Kim wrote:
> >>> On Mon, Nov 16, 2020 at 03:19:41PM +0300, Alexey Budankov wrote:
> >>
> >> <SNIP>
> >>
> >>>>
> >>>> @@ -1400,8 +1417,12 @@ static int record__mmap_read_evlist(struct record *rec, struct evlist *evlist,
> >>>>      /*
> >>>>       * Mark the round finished in case we wrote
> >>>>       * at least one event.
> >>>> +     *
> >>>> +     * No need for round events in directory mode,
> >>>> +     * because per-cpu maps and files have data
> >>>> +     * sorted by kernel.
> >>>
> >>> But it's not just for single cpu since task can migrate so we need to
> >>> look at other cpu's data too.  Thus we use the ordered events queue
> >>> and round events help to determine when to flush the data.  Without
> >>> the round events, it'd consume huge amount of memory during report.
> >>>
> >>> If we separate tracking records and process them first, we should be
> >>> able to process samples immediately without sorting them in the
> >>> ordered event queue.  This will save both cpu cycles and memory
> >>> footprint significantly IMHO.
> >>>
> >>> Thanks,
> >>> Namhyung
> >>>
> >>
> >> As far as I understand, to split tracing records (FORK/MMAP/COMM) into
> >> a separate file, we need to implement a runtime trace decoder on the
> >> perf-record side to recognize such tracing records coming from the kernel.
> >> Is that what you mean?
> >
> > No, I meant separating the mmap buffers so that the record process
> > can save the data without decoding.
> >
>
> Thanks,
>
> Do you think this can be implemented only on the user side by creating a dummy
> event and manipulating by mmap/comm/task flags of struct perf_event_attr?
> Or some changes on the kernel side are necessary?

It's only user space changes but it can be large.  Actually I worked
on parallelizing
perf report several years ago (not finished, but I don't have time for
it now).  At the
time, perf record didn't support directory output so I made it have indexes to
different data parts. But you can get the idea from the code in

  https://git.kernel.org/pub/scm/linux/kernel/git/namhyung/linux-perf.git/log/?h=perf/threaded-v5

Thanks,
Namhyung