Re: [PATCH dwarves v3 3/3] dwarf_loader: permit merging all dwarf cu's for clang lto built binary

[Bill Wendling <morbo@google.com>]

On Tue, Mar 30, 2021 at 1:15 PM Yonghong Song <yhs@fb.com> wrote:
> On 3/30/21 1:08 PM, Bill Wendling wrote:
> > On Sun, Mar 28, 2021 at 1:14 PM Yonghong Song <yhs@fb.com> wrote:
> >>
> >> For vmlinux built with clang thin-lto or lto, there exist
> >> cross cu type references. For example, the below can happen:
> >>    compile unit 1:
> >>       tag 10:  type A
> >>    compile unit 2:
> >>       ...
> >>         refer to type A (tag 10 in compile unit 1)
> >> I only checked a few but have seen type A may be a simple type
> >> like "unsigned char" or a complex type like an array of base types.
> >>
> >> To resolve this issue, the tag DW_AT_producer of the first
> >> DW_TAG_compile_unit is checked. If the binary is built
> >> with clang lto, all debuginfo dwarf cu's will be merged
> >> into one pahole cu which will resolve the above
> >> cross-cu tag reference issue. To test whether a binary
> >> is built with clang lto or not, The "clang version"
> >> and "-flto" will be checked against DW_AT_producer string
> >> for the first 5 debuginfo cu's. The reason is that
> >> a few linux files disabled lto for various reasons.
> >>
> >> Merging cu's will create a single cu with lots of types, tags
> >> and functions. For example with clang thin-lto built vmlinux,
> >> I saw 9M entries in types table, 5.2M in tags table. The
> >> below are pahole wallclock time for different hashbits:
> >> command line: time pahole -J vmlinux
> >>        # of hashbits            wallclock time in seconds
> >>            15                       460
> >>            16                       255
> >>            17                       131
> >>            18                       97
> >>            19                       75
> >>            20                       69
> >>            21                       64
> >>            22                       62
> >>            23                       58
> >>            24                       64
> >>
> >> The problem is with hashtags__find(), esp. the loop
> >>      uint32_t bucket = hashtags__fn(id);
> >>      const struct hlist_head *head = hashtable + bucket;
> >>      hlist_for_each_entry(tpos, pos, head, hash_node) {
> >>              if (tpos->id == id)
> >>                      return tpos;
> >>      }
> >>
> >> Say we have 9M types and (1 << 15) buckets, that means each bucket
> >> will have roughly 64 elements. So each lookup will traverse
> >> the loop 32 iterations on average.
> >>
> >> If we have 1 << 21 buckets, then each buckets will have 4 elements,
> >> and the average number of loop iterations for hashtags__find()
> >> will be 2.
> >>
> >> Note that the number of hashbits 24 makes performance worse
> >> than 23. The reason could be that 23 hashbits can cover 8M
> >> buckets (close to 9M for the number of entries in types table).
> >> Higher number of hash bits allocates more memory and becomes
> >> less cache efficient compared to 23 hashbits.
> >>
> >> This patch picks # of hashbits 21 as the starting value
> >> and will try to allocate memory based on that, if memory
> >> allocation fails, we will go with less hashbits until
> >> we reach hashbits 15 which is the default for
> >> non merge-cu case.
> >>
> >> Signed-off-by: Yonghong Song <yhs@fb.com>
> >> ---
> >>   dwarf_loader.c | 120 +++++++++++++++++++++++++++++++++++++++++++++++++
> >>   1 file changed, 120 insertions(+)
> >>
> >> diff --git a/dwarf_loader.c b/dwarf_loader.c
> >> index aa6372a..a51391e 100644
> >> --- a/dwarf_loader.c
> >> +++ b/dwarf_loader.c
> >> @@ -51,6 +51,7 @@ struct strings *strings;
> >>   #endif
> >>
> >>   static uint32_t hashtags__bits = 15;
> >> +static uint32_t max_hashtags__bits = 21;
> >>
> >>   static uint32_t hashtags__fn(Dwarf_Off key)
> >>   {
> >> @@ -2484,6 +2485,115 @@ static int cus__load_debug_types(struct cus *cus, struct conf_load *conf,
> >>          return 0;
> >>   }
> >>
> >> +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) {
> >> +               Dwarf_Die die_mem;
> >> +               Dwarf_Die *cu_die = dwarf_offdie(dw, off + cuhl, &die_mem);
> >> +
> >> +               if (cu_die == NULL)
> >> +                       break;
> >> +
> >> +               if (++cnt > 5)
> >> +                       break;
> >> +
> >> +               const char *producer = attr_string(cu_die, DW_AT_producer);
> >> +               if (strstr(producer, "clang version") != NULL &&
> >> +                   strstr(producer, "-flto") != NULL)
> >
> > Instead of checking for flags, which can be a bit brittle, would it
> > make more sense to scan the abbreviations to see if there are any
> > "sec_offset" encodings used for type attributes to indicate that LTO
> > was used?
>
> Do you have additional info related to "sec_offset"? I scanned through
> my llvm-dwarfdump result and didn't find it.
>
Sorry about that. It was the wrong thing to check. I consulted our
DWARF expert here and he said this.

"DW_FORM_ref_addr is the important thing used for cross-CU references,
like in the DW_AT_abstract_origin of the DW_TAG_inlined_subroutine. In
intra-CU references, DW_FORM_ref4 is used instead."

-bw

> >
> > Thank you for improving on my hacky patch! :-)
> >
> > -bw
> >
> >> +                       return true;
> >> +
> >> +               off = noff;
> >> +       }
> >> +
> >> +       return false;
> >> +}
> >> +
> [...]