Re: [PATCH v2 1/2] perf record: Add 8-byte aligned event type PERF_RECORD_COMPRESSED2

Arnaldo Carvalho de Melo
2025-03-17T20:24:03Z

On Mon, Mar 17, 2025 at 04:36:05PM -0300, Arnaldo Carvalho de Melo wrote:
> On Mon, Mar 17, 2025 at 09:32:46AM -0700, Ian Rogers wrote:
> > On Mon, Mar 17, 2025 at 9:17 AM Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
> > > On Mon, Mar 17, 2025 at 12:52:09PM -0300, Arnaldo Carvalho de Melo wrote:
> > > > On Fri, Mar 14, 2025 at 06:27:05PM -0700, Namhyung Kim wrote:
> > > > > On Mon, Mar 03, 2025 at 10:32:40AM -0800, Chun-Tse Shao wrote:
> > > > > > The original PERF_RECORD_COMPRESS is not 8-byte aligned, which can cause
> > > > > > asan runtime error:
> > >
> > > > > > # Build with asan
> > > > > > $ make -C tools/perf O=/tmp/perf DEBUG=1 EXTRA_CFLAGS="-O0 -g -fno-omit-frame-pointer -fsanitize=undefined"
> > > > > > # Test success with many asan runtime errors:
> > > > > > $ /tmp/perf/perf test "Zstd perf.data compression/decompression" -vv
> > > > > > 83: Zstd perf.data compression/decompression:
> > > > > > ...
> > > > > > util/session.c:1959:13: runtime error: member access within misaligned address 0x7f69e3f99653 for type 'union perf_event', which requires 13 byte alignment
> > > > > > 0x7f69e3f99653: note: pointer points here
> > > > > > d0 3a 50 69 44 00 00 00 00 00 08 00 bb 07 00 00 00 00 00 00 44 00 00 00 00 00 00 00 ff 07 00 00
> > > > > > ^
> > > > > > util/session.c:2163:22: runtime error: member access within misaligned address 0x7f69e3f99653 for type 'union perf_event', which requires 8 byte alignment
> > > > > > 0x7f69e3f99653: note: pointer points here
> > > > > > d0 3a 50 69 44 00 00 00 00 00 08 00 bb 07 00 00 00 00 00 00 44 00 00 00 00 00 00 00 ff 07 00 00
> > > > > > ^
> > > > > > ...
> > >
> > > > > > Since there is no way to align compressed data in zstd compression, this
> > > > > > patch add a new event type `PERF_RECORD_COMPRESSED2`, which adds a field
> > > > > > `data_size` to specify the actual compressed data size. The
> > > > > > `header.size` contains the total record size, including the padding at
> > > > > > the end to make it 8-byte aligned.
> > >
> > > > > > Tested with `Zstd perf.data compression/decompression`
> > >
> > > > > Looks good to me.
> > >
> > > > > Arnaldo, are you ok with adding a new record type for this?
> > >
> > > > Checking the discussion and the patch.
> > >
> > > My first impression yesterday when I saw this on the smartphone was: how
> > > will an old perf binary handle the new PERF_RECORD_COMPRESSED2? Will it
> > > ignore it while emitting a warning, since it can be skipped and then
> > > what we will get a partial view?
> > >
> > > Having some session output showing how an older perf binary handles
> > > PERF_RECORD_COMPRESS2 would be informative.
> > >
> > > I'll try to reproduce/test this all...
> >
> > I'm not sure we've worried about old perfs being able to read new
> > perf.data files, but we've worried about new perfs being able to read
> > old perf.data files. So if a change is additive, which this change is,
> > then nothing should be impacted.
>
> Right, its difficult to make it work both ways, even with testing, but
> by 'work' I mean that new stuff should be ignored by older versions,
> i.e. records skipped and then the results will surely be different.
>
> So I'm just curious how older tools will handle these new files and to,
> if not that super difficult, to improve how we handle unknown records so
> that in the future, when we add new stuff, we mention that this is
> something not handled, please use a new tool.
>
> > My thoughts are this way as this patch:
> > https://lore.kernel.org/all/20220614143353.1559597-7-irogers@google.com/
> > changed most perf.data cpumap encodings in a way that old perfs won't
> > be able to handle.
>
> > Perhaps testing/documentation should be present for this kind of thing.
>
> Right, but in this specific case it should be a matter of telling the
> user that the header.type PERF_RECORD_COMPRESS2 isn't supported and that
> the user should try and update their tool.
>
> But now back to figuring out how to generate PERF_RECORD_COMPRESS is
> generated, use it, then apply this patch, and then see if how the old
> tool copes.

Which is present in this series, we have even a perf test shell for it,
so happy to not having figured that before :-)

# Build with asan
$ make -C tools/perf O=/tmp/perf DEBUG=1 EXTRA_CFLAGS="-O0 -g -fno-omit-frame-pointer -fsanitize=undefined"
# Test success with many asan runtime errors:
$ /tmp/perf/perf test "Zstd perf.data compression/decompression" -vv
83: Zstd perf.data compression/decompression:

> So documentation is also lacking in suggesting that enumerating the
> steps needed to test before/after is greatly appreciated.
>
> - Arnaldo