Messages in this thread | | | From | Jiri Olsa <> | Date | Fri, 29 Jul 2022 13:35:48 +0200 | Subject | Re: [PATCH v2 0/6] Corrections to cpu map event encoding |
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On Thu, Jul 28, 2022 at 07:01:09PM -0700, Ian Rogers wrote: > 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.
hi, there's some unanswered feedback: https://lore.kernel.org/linux-perf-users/YrwY3SP+jsTwrRBw@krava/
jirka
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