Re: [PATCH v2 0/6] Corrections to cpu map event encoding

[Ian Rogers <irogers@google.com>]

On Tue, Jun 14, 2022 at 7:33 AM Ian Rogers <irogers@google.com> wrote:
>
> A mask encoding of a cpu map is laid out as:
>   u16 nr
>   u16 long_size
>   unsigned long mask[];
> However, the mask may be 8-byte aligned meaning there is a 4-byte pad
> after long_size. This means 32-bit and 64-bit builds see the mask as
> being at different offsets. On top of this the structure is in the byte
> data[] encoded as:
>   u16 type
>   char data[]
> This means the mask's struct isn't the required 4 or 8 byte aligned, but
> is offset by 2. Consequently the long reads and writes are causing
> undefined behavior as the alignment is broken.
>
> These changes do minor clean up with const, visibility of functions
> and using the constant time max function. It then adds 32 and 64-bit
> mask encoding variants, packed to match current alignment. Taking the
> address of a packed struct leads to unaligned data, so function
> arguments are altered to be passed the packed struct. To compact the
> mask encoding further and drop the padding, the 4-byte variant is
> preferred. Finally a new range encoding is added, that reduces the
> size of the common case of a range of CPUs to a single u64.
>
> On a 72 CPU (hyperthread) machine the original encoding of all CPUs is:
> 0x9a98 [0x28]: event: 74
> .
> . ... raw event: size 40 bytes
> .  0000:  4a 00 00 00 00 00 28 00 01 00 02 00 08 00 00 00  J.....(.........
> .  0010:  00 00 ff ff ff ff ff ff ff ff ff 00 00 00 00 00  ................
> .  0020:  00 00 00 00 00 00 00 00                          ........
>
> 0 0 0x9a98 [0x28]: PERF_RECORD_CPU_MAP
>
> Using the 4-byte encoding it is:
> 0x9a98@pipe [0x20]: event: 74
> .
> . ... raw event: size 32 bytes
> .  0000:  4a 00 00 00 00 00 20 00 01 00 03 00 04 00 ff ff  J..... .........
> .  0010:  ff ff ff ff ff ff ff 00 00 00 00 00 00 00 00 00  ................
>
> 0 0 0x9a98 [0x20]: PERF_RECORD_CPU_MAP
>
> Finally, with the range encoding it is:
> 0x9ab8@pipe [0x10]: event: 74
> .
> . ... raw event: size 16 bytes
> .  0000:  4a 00 00 00 00 00 10 00 02 00 00 00 00 00 47 00  J.............G.
>
> 0 0 0x9ab8 [0x10]: PERF_RECORD_CPU_MAP
>
> v2. Fixes a bug in the size computation of the update header
>     introduced by the last patch (Add range data encoding) and caught
>     by address sanitizer.
>
> Ian Rogers (6):
>   perf cpumap: Const map for max
>   perf cpumap: Synthetic events and const/static
>   perf cpumap: Compute mask size in constant time
>   perf cpumap: Fix alignment for masks in event encoding
>   perf events: Prefer union over variable length array
>   perf cpumap: Add range data encoding

Ping. There was some feedback on this change but nothing to create a
v3. Feedback/acked-by/reviewed-by appreciated.

Thanks,
Ian

>  tools/lib/perf/cpumap.c              |   2 +-
>  tools/lib/perf/include/perf/cpumap.h |   2 +-
>  tools/lib/perf/include/perf/event.h  |  61 ++++++++-
>  tools/perf/tests/cpumap.c            |  71 ++++++++---
>  tools/perf/tests/event_update.c      |  14 +--
>  tools/perf/util/cpumap.c             | 111 +++++++++++++---
>  tools/perf/util/cpumap.h             |   4 +-
>  tools/perf/util/event.h              |   4 -
>  tools/perf/util/header.c             |  24 ++--
>  tools/perf/util/session.c            |  35 +++---
>  tools/perf/util/synthetic-events.c   | 182 +++++++++++++--------------
>  tools/perf/util/synthetic-events.h   |   2 +-
>  12 files changed, 327 insertions(+), 185 deletions(-)
>
> --
> 2.36.1.476.g0c4daa206d-goog
>