Em Wed, Aug 05, 2020 at 10:07:58AM -0700, Hao Luo escreveu:
> Hi, Arnaldo,
>
> Friendly ping. Do you have a timeline for merging this feature? Is
> there anything that I can help?
I had already merged it, but wasn't in the master branch, it is now, I'm
finishing some other feature to then cut v1.18,
Thanks,
- Arnaldo
> Thanks,
> Hao
>
>
> On Thu, Jul 9, 2020 at 11:17 AM Hao Luo <haoluo@xxxxxxxxxx> wrote:
> >
> > Sounds good. Thanks!
> >
> > On Thu, Jul 9, 2020 at 8:41 AM Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> wrote:
> > >
> > > Em Wed, Jul 08, 2020 at 01:56:02PM -0700, Andrii Nakryiko escreveu:
> > > > On Wed, Jul 8, 2020 at 1:44 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
> > > > >
> > > > > On SMP systems, the global percpu variables are placed in a special
> > > > > '.data..percpu' section, which is stored in a segment whose initial
> > > > > address is set to 0, the addresses of per-CPU variables are relative
> > > > > positive addresses [1].
> > > > >
> > > > > This patch extracts these variables from vmlinux and places them with
> > > > > their type information in BTF. More specifically, when BTF is encoded,
> > > > > we find the index of the '.data..percpu' section and then traverse
> > > > > the symbol table to find those global objects which are in this section.
> > > > > For each of these objects, we push a BTF_KIND_VAR into the types buffer,
> > > > > and a BTF_VAR_SECINFO into another buffer, percpu_secinfo. When all the
> > > > > CUs have finished processing, we push a BTF_KIND_DATASEC into the
> > > > > btfe->types buffer, followed by the percpu_secinfo's content.
> > > > >
> > > > > In a v5.8-rc3 linux kernel, I was able to extract 288 such variables.
> > > > > The build time overhead is small and the space overhead is also small.
> > > > > See testings below.
> > > > >
> > > > > A found variable can be invalid in two ways:
> > > > >
> > > > > - Its name found in elf_sym__name is invalid.
> > > > > - Its size identified by elf_sym__size is 0.
> > > > >
> > > > > In either case, the btf containing such symbols will be rejected by the
> > > > > btf verifier. Normally we should not see such symbols. But if one is
> > > > > seen during btf encoding, the encoder will exit with error. An new flag
> > > > > '-j' (or '--force') is implemented to help testing, which skips the
> > > > > invalid symbols and force emit a btf.
> > > > >
> > > > > Testing:
> > > > >
> > > > > - Vmlinux size has increased by ~12kb.
> > > > > Before:
> > > > > $ readelf -SW vmlinux | grep BTF
> > > > > [25] .BTF PROGBITS ffffffff821a905c 13a905c 2d2bf8 00
> > > > > After:
> > > > > $ pahole -J vmlinux
> > > > > $ readelf -SW vmlinux | grep BTF
> > > > > [25] .BTF PROGBITS ffffffff821a905c 13a905c 2d5bca 00
> > > > >
> > > > > - Common global percpu VARs and DATASEC are found in BTF section.
> > > > > $ bpftool btf dump file vmlinux | grep runqueues
> > > > > [14152] VAR 'runqueues' type_id=13778, linkage=global-alloc
> > > > >
> > > > > $ bpftool btf dump file vmlinux | grep 'cpu_stopper'
> > > > > [17582] STRUCT 'cpu_stopper' size=72 vlen=5
> > > > > [17601] VAR 'cpu_stopper' type_id=17582, linkage=static
> > > > >
> > > > > $ bpftool btf dump file vmlinux | grep ' DATASEC '
> > > > > [63652] DATASEC '.data..percpu' size=179288 vlen=288
> > > > >
> > > > > - Tested bpf selftests.
> > > > >
> > > > > - Pahole exits with error if an invalid symbol is seen during encoding,
> > > > > make -f Makefile -j 36 -s
> > > > > PAHOLE: Error: Found symbol of zero size when encoding btf (sym: 'yyy', cu: 'xxx.c').
> > > > > PAHOLE: Error: Use '-j' or '--force_emit' to ignore such symbols and force emit the btf.
> > > > > scripts/link-vmlinux.sh: line 137: 2475712 Segmentation fault LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
> > > > >
> > > > > - With the flag '-j' or '--force', the invalid symbols are ignored.
> > > > >
> > > > > - Further in verbose mode and with '-j' or '--force' set, a warning is generated:
> > > > > PAHOLE: Warning: Found symbol of zero size when encoding btf, ignored (sym: 'yyy', cu: 'xxx.c').
> > > > > PAHOLE: Warning: Found symbol of invalid name when encoding btf, ignored (sym: 'zzz', cu: 'sss.c').
> > > > >
> > > > > References:
> > > > > [1] https://lwn.net/Articles/531148/
> > > > >
> > > > > Acked-by: Andrii Nakryiko <andriin@xxxxxx>
> > > > > Tested-by: Andrii Nakryiko <andriin@xxxxxx>
> > > > > Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> > > > > ---
> > > >
> > > > Looks great, thanks!
> > >
> > > The only thing I didn't like was to grab a one letter option for
> > > something that should be seldomly used, so I'll make it be just --force,
> > > leaving -j for future usage,
> > >
> > > I'll test this, make this change and apply, probably this, together with
> > > some pretty printing features I'm working on are enough for a new
> > > release.
> > >
> > > Thanks,
> > >
> > > - Arnaldo
--
- Arnaldo
