Re: [PATCH 2/2] btf_encoder: Fix function generation

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Mon, Nov 16, 2020 at 04:15:44PM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Mon, Nov 16, 2020 at 07:21:45PM +0100, Jiri Olsa escreveu:
> > On Mon, Nov 16, 2020 at 10:50:16AM -0300, Arnaldo Carvalho de Melo wrote:
> > > Em Fri, Nov 13, 2020 at 01:43:47PM -0800, Andrii Nakryiko escreveu:
> > > > On Fri, Nov 13, 2020 at 1:29 PM Jiri Olsa <jolsa@redhat.com> wrote:
> > > 
> > > > > On Fri, Nov 13, 2020 at 12:56:40PM -0800, Andrii Nakryiko wrote:
> > > > > > On Fri, Nov 13, 2020 at 7:13 AM Jiri Olsa <jolsa@kernel.org> wrote:
> > > 
> > > > > > > Current conditions for picking up function records break
> > > > > > > BTF data on some gcc versions.
> > > 
> > > > > > > Some function records can appear with no arguments but with
> > > > > > > declaration tag set, so moving the 'fn->declaration' in front
> > > > > > > of other checks.
> > > 
> > > > > > > Then checking if argument names are present and finally checking
> > > > > > > ftrace filter if it's present. If ftrace filter is not available,
> > > > > > > using the external tag to filter out non external functions.
> > > 
> > > > > > > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > > 
> > > > > > I tested locally, all seems to work fine. Left few suggestions below,
> > > > > > but those could be done in follow ups (or argued to not be done).
> > > 
> > > > > > Acked-by: Andrii Nakryiko <andrii@kernel.org>
> > > 
> > > > > > BTW, for some stats.
> > > 
> > > > > > BEFORE allowing static funcs:
> > > 
> > > 
> > > Nowhere in the last patchkit comments is some explanation for the
> > > inclusion of static functions :-\ After the first patch in the last
> > > series I get:
> > > 
> > >   $ llvm-objcopy --remove-section=.BTF vmlinux
> > >   $ readelf -SW vmlinux  | grep BTF
> > >   $ pahole -J vmlinux
> > >   $ bpftool btf dump file ./vmlinux | grep 'FUNC '| cut -d\' -f2 | sort > before.bpftool
> > >   $ cp vmlinux vmlinux.before.all
> > >   $ wc -l before.bpftool
> > >   28829 before.bpftool
> > 
> > I think you see the original number of functions, because without
> > the 'not merged' kernel patch, that added the special init section,
> > pahole will fail to detect vmlinux and fall back to checking dwarf
> > declarations
> 
> Indeed, I moved the verbose/force setting to the beggining of the
> encoder and:
> 
> ------------
> Found 352 per-CPU variables!
> vmlinux not detected, falling back to dwarf data
> File vmlinux:
> search cu '/home/acme/git/linux/arch/x86/kernel/head_64.S' for percpu global variables.
> -----------------
> 
> Now I have to read that code to figure out what that "vmlinux not
> detected, falling back to dwarf data" message means, as vmlinux is where
> DWARF data is, so what is that isn't being "detected", /me checks...

So with some debugging I see, the message is just confusing:

"vmlinux not detected, falling back to dwarf data (functions_cnt=53238, has_all_symbols(&fl)=0"

It finds the ELF symtab, finds the percpu variables there, tons of
functions, matching the number after this approach of marking BPF init
functions was dropped its just that vague "has_all_symbols()" routine
that fails to find all the symbols it needs in the vmlinux file.

- Arnaldo