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?
Thank you for improving on my hacky patch! :-)
-bw
> + return true;
> +
> + off = noff;
> + }
> +
> + return false;
> +}
> +
> +static int cus__merge_and_process_cu(struct cus *cus, struct conf_load *conf,
> + Dwfl_Module *mod, Dwarf *dw, Elf *elf,
> + const char *filename,
> + const unsigned char *build_id,
> + int build_id_len,
> + struct dwarf_cu *type_dcu)
> +{
> + uint8_t pointer_size, offset_size;
> + struct dwarf_cu *dcu = NULL;
> + Dwarf_Off off = 0, noff;
> + struct cu *cu = NULL;
> + size_t cuhl;
> +
> + 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 (cu == NULL) {
> + cu = cu__new("", pointer_size, build_id, build_id_len,
> + filename);
> + if (cu == NULL || cu__set_common(cu, conf, mod, elf) != 0)
> + return DWARF_CB_ABORT;
> +
> + dcu = malloc(sizeof(struct dwarf_cu));
> + if (dcu == NULL)
> + return DWARF_CB_ABORT;
> +
> + /* Merged cu tends to need a lot more memory.
> + * Let us start with max_hashtags__bits and
> + * go down to find a proper hashtag bit value.
> + */
> + uint32_t default_hbits = hashtags__bits;
> + for (hashtags__bits = max_hashtags__bits;
> + hashtags__bits >= default_hbits;
> + hashtags__bits--) {
> + if (dwarf_cu__init(dcu) == 0)
> + break;
> + }
> + if (hashtags__bits < default_hbits)
> + return DWARF_CB_ABORT;
> +
> + dcu->cu = cu;
> + dcu->type_unit = type_dcu;
> + cu->priv = dcu;
> + cu->dfops = &dwarf__ops;
> + cu->language = attr_numeric(cu_die, DW_AT_language);
> + }
> +
> + Dwarf_Die child;
> + if (dwarf_child(cu_die, &child) == 0) {
> + if (die__process_unit(&child, cu) != 0)
> + return DWARF_CB_ABORT;
> + }
> +
> + off = noff;
> + }
> +
> + /* process merged cu */
> + if (cu__recode_dwarf_types(cu) != LSK__KEEPIT)
> + return DWARF_CB_ABORT;
> + if (finalize_cu_immediately(cus, cu, dcu, conf)
> + == LSK__STOP_LOADING)
> + return DWARF_CB_ABORT;
> +
> + return 0;
> +}
> +
> static int cus__load_module(struct cus *cus, struct conf_load *conf,
> Dwfl_Module *mod, Dwarf *dw, Elf *elf,
> const char *filename)
> @@ -2518,6 +2628,15 @@ static int cus__load_module(struct cus *cus, struct conf_load *conf,
> }
> }
>
> + if (cus__merging_cu(dw)) {
> + res = cus__merge_and_process_cu(cus, conf, mod, dw, elf, filename,
> + build_id, build_id_len,
> + type_cu ? &type_dcu : NULL);
> + if (res)
> + return res;
> + goto out;
> + }
> +
> while (dwarf_nextcu(dw, off, &noff, &cuhl, NULL, &pointer_size,
> &offset_size) == 0) {
> Dwarf_Die die_mem;
> @@ -2557,6 +2676,7 @@ static int cus__load_module(struct cus *cus, struct conf_load *conf,
> off = noff;
> }
>
> +out:
> if (type_lsk == LSK__DELETE)
> cu__delete(type_cu);
>
> --
> 2.30.2
>
