linux-kernel - Re: [PATCH] perf inject: Flush ordered events on FINISHED

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Message-ID: <CAM9d7cgsxkefHAgyMf-GoP4-OdSsaRmhSGLwPMoYn=-c9YXxDw@mail.gmail.com>
Date:   Tue, 6 Oct 2020 10:28:35 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Ian Rogers <irogers@...gle.com>,
        Al Grant <al.grant@...s.arm.com>,
        Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: [PATCH] perf inject: Flush ordered events on FINISHED_ROUND

Hi Jiri,

On Mon, Oct 5, 2020 at 4:52 AM Jiri Olsa <jolsa@...hat.com> wrote:
>
> On Fri, Oct 02, 2020 at 10:03:17PM +0900, Namhyung Kim wrote:
> > Currently perf inject just repipes the event without any flush.  It
> > makes an issue that it changes the order of events processed.
> >
> > Normally we want to process events in time order, but events are
> > recorded from several cpus and they weren't sorted with each other.
> > So we put them into the ordered event queue, sorted by time, and flush
> > them when we see a next FINISHED_ROUND event.
> >
> > But this is for events from kernel, user space events (like the
> > FINISHED_ROUND) are processed without queueing.  So during the perf
> > inject, it writes all the FINISHED_ROUND events directly while
> > SAMPLE (and other) events are kept in the queue.  This makes the user
> > space events are put before any kernel events.
> >
> > You can see that with the following command:
> >
> >   $ perf record -a -o- sleep 1 | perf inject -b | perf script -i- --show-round-events
> >   PERF_RECORD_FINISHED_ROUND
> >   PERF_RECORD_FINISHED_ROUND
> >   PERF_RECORD_FINISHED_ROUND
> >   ...
> >
> > Omitting perf inject, you can see the events are placed in the middle
> > of the data.
> >
> > You might argue that the whole point of the FINISHED_ROUND event is to
> > sort (kernel) events.  And after perf inject, all events are written
> > in a proper time order so we don't care about the FINISHED_ROUND event
> > anymore.
> >
> > But the problem is we don't know whether the input data is sorted or
> > not (maybe we can add a feature bit for this?) so it should use an
> > ordered event queue when processing the input like in perf report.
>
> I like the idea of storing the information that the data is sorted,
> and when it's there, let's not use ordered_oevets

Thanks for your input.  Yeah, I think it's better not to use it if possible.

>
> >
> > Remember all the FINISHED_ROUND events now come before other events so
> > the tool cannot know when it can flush the data so everything will be
> > queued until it meets the end of the input.  Actually it's same for
> > perf inject itself as it doesn't flush the queue.
> >
> > Below measures time and memory usage during the perf inject and
> > report using ~190MB data file.
> >
> > Before:
> >   perf inject:  11.09 s,  382148 KB
> >   perf report:   8.05 s,  397440 KB
> >
> > After:
> >   perf inject:  16.24 s,   83376 KB
> >   perf report:   7.96 s,  216184 KB
> >
> > As you can see, it used 2x memory of the input size.  I guess it's
> > because it needs to keep the copy for the whole input.  But I don't
> > understand why processing time of perf inject increased..
>
> would be good to find out first

Will do!

Thanks
Namhyung