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    SubjectRe: [PATCH v2 0/6] Corrections to cpu map event encoding
    On Fri, Jul 29, 2022 at 4:35 AM Jiri Olsa <olsajiri@gmail.com> wrote:
    >
    > 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

    Thanks Jirka,

    Was there a comment in particular? My reply was here:
    https://lore.kernel.org/linux-perf-users/CAP-5=fU=BpP4OT2axZLSfRnKxQxmv-HXj8khBgmx3XQMS+abgA@mail.gmail.com/
    I double checked, everyone of your comments was answered.

    Thanks,
    Ian

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