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dwarf_loader: Check .debug_abbrev for cross-CU references 

Commit 39227909db checked compilation flags to see whether the binary
is built with LTO or not (-flto).

Currently, for clang LTO build, default setting won't put compilation
flags in dwarf due to size concern.

David Blaikie suggested in [1] to scan through .debug_abbrev for
DW_FORM_ref_addr which should be most faster than scanning through CU's.

This patch implemented this suggestion and replaced the previous
compilation flag matching approach. Indeed, it seems that the overhead
for this approach is indeed managable.

I did some change to measure the overhead of cus_merging_cu():
  @@ -2650,7 +2652,15 @@ static int cus__load_module(struct cus *cus, struct conf_load *conf,
                  }
          }

  -       if (cus__merging_cu(dw)) {
  +       bool do_merging;
  +       struct timeval start, end;
  +       gettimeofday(&start, NULL);
  +       do_merging = cus__merging_cu(dw);
  +       gettimeofday(&end, NULL);
  +       fprintf(stderr, "%ld %ld -> %ld %ld\n", start.tv_sec, start.tv_usec,
  +                       end.tv_sec, end.tv_usec);
  +
  +       if (do_merging) {
                  res = cus__merge_and_process_cu(cus, conf, mod, dw, elf, filename,
                                                  build_id, build_id_len,
                                                  type_cu ? &type_dcu : NULL);

For LTO vmlinux, the cus__merging_cu() checking takes 130us over total
"pahole -J vmlinux" time 65sec as the function bail out earlier due to
detecting a merging CU condition.

For non-LTO vmlinux, the cus__merging_cu() checking takes ~171368us over
total pahole time 36sec, roughly 0.5% overhead.

 [1] https://lore.kernel.org/bpf/20210328064121.2062927-1-yhs@fb.com/

Suggested-by: David Blaikie <blaikie@google.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Bill Wendling <morbo@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: David Blaikie <dblaikie@gmail.com>
Cc: Fāng-ruì Sòng <maskray@google.com>
Cc: bpf@vger.kernel.org
Cc: dwarves@vger.kernel.org
Cc: kernel-team@fb.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
This commit is contained in:
Yonghong Song 2021-03-31 19:58:20 -07:00 committed by Arnaldo Carvalho de Melo
parent 39227909db
commit 209e45424f
1 changed files with 24 additions and 19 deletions
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43
dwarf_loader.c
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@ -2503,35 +2503,40 @@ static int cus__load_debug_types(struct cus *cus, struct conf_load *conf,
static bool cus__merging_cu(Dwarf *dw)
{
uint8_t pointer_size, offset_size;
Dwarf_Off off = 0, noff;
size_t cuhl;
int cnt = 0;
/*
* Just checking the first cu is not enough.
* In linux, some C files may have LTO is disabled, e.g.,
* e242db40be27 x86, vdso: disable LTO only for vDSO
* d2dcd3e37475 x86, cpu: disable LTO for cpu.c
* Fortunately, disabling LTO for a particular file in a LTO build
* is rather an exception. Iterating 5 cu's to check whether
* LTO is used or not should be enough.
*/
while (dwarf_nextcu(dw, off, &noff, &cuhl, NULL, &pointer_size,
&offset_size) == 0) {
while (dwarf_nextcu (dw, off, &noff, &cuhl, NULL, NULL, NULL) == 0) {
Dwarf_Die die_mem;
Dwarf_Die *cu_die = dwarf_offdie(dw, off + cuhl, &die_mem);
if (cu_die == NULL)
break;
if (++cnt > 5)
break;
Dwarf_Off offset = 0;
while (true) {
size_t length;
Dwarf_Abbrev *abbrev = dwarf_getabbrev (cu_die, offset, &length);
if (abbrev == NULL || abbrev == DWARF_END_ABBREV)
break;
const char *producer = attr_string(cu_die, DW_AT_producer);
if (strstr(producer, "clang version") != NULL &&
strstr(producer, "-flto") != NULL)
return true;
size_t attrcnt;
if (dwarf_getattrcnt (abbrev, &attrcnt) != 0)
return false;
unsigned int attr_num, attr_form;
Dwarf_Off aboffset;
size_t j;
for (j = 0; j < attrcnt; ++j) {
if (dwarf_getabbrevattr (abbrev, j, &attr_num, &attr_form,
&aboffset))
return false;
if (attr_form == DW_FORM_ref_addr)
return true;
}
offset += length;
}
off = noff;
}