linux-kernel - Re: [PATCH 3/4] perf hist: Do not use event index in hpp_

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Message-ID: <CAM9d7cg6c7t7zAgDEJP=+D1qM16SQuWN05fn4T-m1WPfXjaZvA@mail.gmail.com>
Date: Fri, 16 Feb 2024 12:08:47 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>, Jiri Olsa <jolsa@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	linux-perf-users@...r.kernel.org
Subject: Re: [PATCH 3/4] perf hist: Do not use event index in hpp__fmt()

On Wed, Feb 14, 2024 at 11:54 PM Ian Rogers <irogers@...gle.com> wrote:
>
> On Wed, Feb 14, 2024 at 9:26 PM Namhyung Kim <namhyung@...nel.org> wrote:
> >
> > On Wed, Feb 14, 2024 at 4:08 PM Ian Rogers <irogers@...gle.com> wrote:
> > >
> > > On Mon, Feb 12, 2024 at 11:52 PM Namhyung Kim <namhyung@...nel.org> wrote:
> > > >
> > > > The __hpp__fmt() is to print period values in a hist entry.  It handles
> > > > event groups using linked pair entries.  Until now, it used event index
> > > > to print values of group members.  But we want to make it more robust
> > > > and support groups even if some members in the group were removed.
> > >
> > > I'm unclear how it breaks currently. The evsel idx is set the evlist
> > > nr_entries on creation and not updated by a remove. A remove may
> > > change a groups leader, should the remove also make the next entry's
> > > index idx that of the previous group leader?
> >
> > The evsel__group_idx() returns evsel->idx - leader->idx.
> > If it has a group event {A, B, C} then the index would be 0, 1, 2.
> > If it removes B, the group would be {A, C} with index 0 and 2.
> > The nr_members is 2 now so it cannot use index 2 for C.
> >
> > Note that we cannot change the index of C because some information
> > like annotation histogram relies on the index.
>
> Ugh, the whole index thing is just messy - perhaps these days we could
> have a hashmap with the evsel as a key instead. I remember that I also
> forced the idx here:
> https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/parse-events.c?h=perf-tools-next#n2049
> If it were invariant that the idx were always the position of an event
> in the evlist then I think life would be easier, but that won't help
> for the arrays of counters that need the index to be constant. I guess
> this is why the previous work to do this skipped evsels rather than
> removed them.

Actually I have a patch series to convert the annotation histogram
to a hash map.  It'd reduce memory usage as well.  Will post.

I think removing evsel is not a common operation and should be
done with care.  In this patchset, it removed (dummy) events after
processing all samples.  I can make the code to skip those event
when printing the result but it'd be much easier if it can remove the
unnecessary events.

Thanks,
Namhyung