Re: [PATCH dwarves 0/2] dwarf_loader: improve cus__merging_cu()

[Jiri Olsa <jolsa@redhat.com>]

On Thu, Apr 01, 2021 at 01:56:57PM -0700, Bill Wendling wrote:
> On Thu, Apr 1, 2021 at 12:35 PM Bill Wendling <morbo@google.com> wrote:
> >
> > On Wed, Mar 31, 2021 at 7:58 PM Yonghong Song <yhs@fb.com> wrote:
> > >
> > > Function cus__merging_cu() is introduced in Commit 39227909db3c
> > > ("dwarf_loader: Permit merging all DWARF CU's for clang LTO built
> > > binary") to test whether cross-cu references may happen.
> > > The original implementation anticipates compilation flags
> > > in dwarf, but later some concerns about binary size surfaced
> > > and the decision is to scan .debug_abbrev as a faster way
> > > to check cross-cu references. Also putting a note in vmlinux
> > > to indicate whether lto is enabled for built or not can
> > > provide a much faster way.
> > >
> > > This patch set implemented this two approaches, first
> > > checking the note (in Patch #2), if not found, then
> > > check .debug_abbrev (in Patch #1).
> > >
> > > Yonghong Song (2):
> > >   dwarf_loader: check .debug_abbrev for cross-cu references
> > >   dwarf_loader: check .notes section for lto build info
> > >
> > >  dwarf_loader.c | 76 ++++++++++++++++++++++++++++++++++++--------------
> > >  1 file changed, 55 insertions(+), 21 deletions(-)
> > >
> > With this series of patches, the compilation passes for me with
> > ThinLTO. You may add this if you like:
> >
> > Tested-by: Bill Wendling <morbo@google.com>
> 
> I did notice these warnings following the "pahole -J .tmp_vmlinux.btf"
> command. I don't know the severity of them, but it might be good to
> investigate.
> 
> $ ./tools/bpf/resolve_btfids/resolve_btfids vmlinux
>   BTFIDS  vmlinux
> WARN: multiple IDs found for 'inode': 355, 8746 - using 355

the message means that there are multiple instances of 'inode'
structs in the BTF data with ids 355 and 8746

that can happen when some object/struct name is duplicated in
multiple objects and it's causing two distinct type hierarchies
in BTF data.. like the one we fixes in here:
  f7f2b43eaf6b crypto: bcm - Rename struct device_private to bcm_device_private

if you provide the .config, I can check if I reproduce

jirka

> WARN: multiple IDs found for 'file': 588, 8779 - using 588
> WARN: multiple IDs found for 'path': 411, 8780 - using 411
> WARN: multiple IDs found for 'seq_file': 1414, 8836 - using 1414
> WARN: multiple IDs found for 'vm_area_struct': 538, 8873 - using 538
> WARN: multiple IDs found for 'task_struct': 28, 8880 - using 28
> WARN: multiple IDs found for 'inode': 355, 9484 - using 355
> WARN: multiple IDs found for 'file': 588, 9517 - using 588
> WARN: multiple IDs found for 'path': 411, 9518 - using 411
> WARN: multiple IDs found for 'seq_file': 1414, 9578 - using 1414
> WARN: multiple IDs found for 'vm_area_struct': 538, 9615 - using 538
> WARN: multiple IDs found for 'task_struct': 28, 9622 - using 28
> WARN: multiple IDs found for 'seq_file': 1414, 12223 - using 1414
> WARN: multiple IDs found for 'file': 588, 12237 - using 588
> WARN: multiple IDs found for 'path': 411, 12238 - using 411
> ...
> 
> -bw
>