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
