On Tue, Mar 30, 2021 at 3:28 PM Yonghong Song <yhs@xxxxxx> wrote:
> On 3/30/21 2:44 PM, Bill Wendling wrote:
> > On Tue, Mar 30, 2021 at 1:15 PM Yonghong Song <yhs@xxxxxx> wrote:
> >> On 3/30/21 1:08 PM, Bill Wendling wrote:
> >>> On Sun, Mar 28, 2021 at 1:14 PM Yonghong Song <yhs@xxxxxx> 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@xxxxxx>
> >>>> ---
> >>>> 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."
>
> Thanks. I checked with lto dwarf, it should work. The only drawback is
> that it needs to traverse all DW_TAG_inlined_subroutine
> and DW_TAG_type's in the cu. If nothing found, going to the next.
> This is exactly what I tried to avoid in the beginning, going deep
> in the dwarf cu. I could remember the maximum and minimum of the ref.
> type tags for each cu, if it is beyond the current cu
> range, it means cross-cu access. But I didn't use this approach
> as it is something every pahole user will pay the price regardless
> of whether their binary is lto or not (mostly probably not.)
>
> I checked my current vmlinux-lto. I will be able
> to find a DW_FORM_ref_addr in the 5th cu. May not be too bad.
> We could have a heuristic to only check the first 10 cu's.
> If no cross cu reference, that means not lto. Not perfect.
> But for sure we don't want to go through all cu's just to
> find it is not lto build.
>
> Still feel we should get whether it is a lto built binary
> as soon as possible. With an explicit option, compiler flags
> seem second choice...
>
That's a good point. It sounds like restricting the search to the
first X CUs will be even more brittle than looking at the command line
flags. It's probably fine then to go with what you originally have.
Sorry for the noise.
-bw
