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

[Jiri Olsa]

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@xxxxxxxxxx> wrote:
> >
> > On Tue, Jun 14, 2022 at 7:33 AM Ian Rogers <irogers@xxxxxxxxxx> 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