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

[Arnaldo Carvalho de Melo]

Em Sun, Aug 14, 2022 at 04:05:19PM -0700, Ian Rogers escreveu:
> Hi,
> 
> It looks like we missed this one. Can it get into the next PR as a fix?

I've queued up to 4/6 for perf/urgent, 5 and 6 will go to perf/core once
I merge perf/urgent there, i.e. after I send this to Linus.

- Arnaldo
 
> Thanks,
> Ian
> 
> 
> On Thu, Aug 4, 2022 at 1:23 PM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
> 
> > On Tue, Jun 14, 2022 at 07:33:47AM -0700, Ian Rogers 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
> >
> > Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> >
> > thanks,
> > jirka
> >
> > >
> > >  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
> > >
> >

-- 

- Arnaldo