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

[Ian Rogers <irogers@google.com>]

On Fri, Jul 15, 2022 at 10:01 AM Ian Rogers <irogers@google.com> wrote:
>
> 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.

Ping. Feedback appreciated.

Thanks,
Ian

> 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
> >