Messages in this thread | | | From | Ian Rogers <> | Date | Fri, 15 Jul 2022 10:01:36 -0700 | Subject | Re: [PATCH v2 0/6] Corrections to cpu map event encoding |
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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 >
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