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

[Arnaldo Carvalho de Melo <acme@kernel.org>]

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...

- Arnaldo