On Tue, 23 Jun 2020 at 03:27, Saravana Kannan <sarava...@google.com> wrote:
>
> When loading a module, module_frob_arch_sections() tries to figure out
> the number of PLTs that'll be needed to handle all the RELAs. While
> doing this, it tries to dedupe PLT allocations for multiple
> R_AARCH64_CALL26 relocations to the same symbol. It does the same for
> R_AARCH64_JUMP26 relocations.
>
> To make checks for duplicates easier/faster, it sorts the relocation
> list by type, symbol and addend. That way, to check for a duplicate
> relocation, it just needs to compare with the previous entry.
>
> However, sorting the entire relocation array is unnecessary and
> expensive (O(n log n)) because there are a lot of other relocation types
> that don't need deduping or can't be deduped.
>
> So this commit partitions the array into entries that need deduping and
> those that don't. And then sorts just the part that needs deduping. And
> when CONFIG_RANDOMIZE_BASE is disabled, the sorting is skipped entirely
> because PLTs are not allocated for R_AARCH64_CALL26 and R_AARCH64_JUMP26
> if it's disabled.
>
> This gives significant reduction in module load time for modules with
> large number of relocations with no measurable impact on modules with a
> small number of relocations. In my test setup with CONFIG_RANDOMIZE_BASE
> enabled, these were the results for a few downstream modules:
>
> Module Size (MB)
> wlan 14
> video codec 3.8
> drm 1.8
> IPA 2.5
> audio 1.2
> gpu 1.8
>
> Without this patch:
> Module Number of entries sorted Module load time (ms)
> wlan 243739 283
> video codec 74029 138
> drm 53837 67
> IPA 42800 90
> audio 21326 27
> gpu 20967 32
>
> Total time to load all these module: 637 ms
>
> With this patch:
> Module Number of entries sorted Module load time (ms)
> wlan 22454 61
> video codec 10150 47
> drm 13014 40
> IPA 8097 63
> audio 4606 16
> gpu 6527 20
>
> Total time to load all these modules: 247
>
> Time saved during boot for just these 6 modules: 390 ms
>
> Cc: Ard Biesheuvel <ard.biesheu...@linaro.org>
[I am no longer at Linaro so please don't use my @linaro.org address]
> Signed-off-by: Saravana Kannan <sarava...@google.com>
> ---
>
> v1 -> v2:
> - Provided more details in the commit text
> - Pulled in Will's comments on the coding style
> - Pulled in Ard's suggestion about skipping jumps with the same section
> index (parts of Will's suggested code)
>
> arch/arm64/kernel/module-plts.c | 46 ++++++++++++++++++++++++++++++---
> 1 file changed, 43 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kernel/module-plts.c b/arch/arm64/kernel/module-plts.c
> index 65b08a74aec6..0ce3a28e3347 100644
> --- a/arch/arm64/kernel/module-plts.c
> +++ b/arch/arm64/kernel/module-plts.c
> @@ -253,6 +253,40 @@ static unsigned int count_plts(Elf64_Sym *syms,
> Elf64_Rela *rela, int num,
> return ret;
> }
>
> +static bool branch_rela_needs_plt(Elf64_Sym *syms, Elf64_Rela *rela,
> + Elf64_Word dstidx)
> +{
> +
> + Elf64_Sym *s = syms + ELF64_R_SYM(rela->r_info);
> +
> + if (s->st_shndx == dstidx)
> + return false;
> +
> + return ELF64_R_TYPE(rela->r_info) == R_AARCH64_JUMP26 ||
> + ELF64_R_TYPE(rela->r_info) == R_AARCH64_CALL26;
> +}
> +
> +/* Group branch PLT relas at the front end of the array. */
> +static int partition_branch_plt_relas(Elf64_Sym *syms, Elf64_Rela *rela,
> + int numrels, Elf64_Word dstidx)
> +{
> + int i = 0, j = numrels - 1;
> +
> + if (!IS_ENABLED(CONFIG_RANDOMIZE_BASE))
> + return 0;
> +
> + while (i < j) {
> + if (branch_rela_needs_plt(syms, &rela[i], dstidx))
> + i++;
> + else if (branch_rela_needs_plt(syms, &rela[j], dstidx))
> + swap(rela[i], rela[j]);
Nit: would be slightly better to put
swap(rela[i++], rela[j]);
here so the next iteration of the loop will not call
branch_rela_needs_plt() on rela[i] redundantly. But the current code
is also correct.
> + else
> + j--;
> + }
> +
> + return i;
> +}
> +
> int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
> char *secstrings, struct module *mod)
> {
> @@ -290,7 +324,7 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr
> *sechdrs,
>
> for (i = 0; i < ehdr->e_shnum; i++) {
> Elf64_Rela *rels = (void *)ehdr + sechdrs[i].sh_offset;
> - int numrels = sechdrs[i].sh_size / sizeof(Elf64_Rela);
> + int nents, numrels = sechdrs[i].sh_size / sizeof(Elf64_Rela);
> Elf64_Shdr *dstsec = sechdrs + sechdrs[i].sh_info;
>
> if (sechdrs[i].sh_type != SHT_RELA)
> @@ -300,8 +334,14 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr
> *sechdrs,
> if (!(dstsec->sh_flags & SHF_EXECINSTR))
> continue;
>
> - /* sort by type, symbol index and addend */
> - sort(rels, numrels, sizeof(Elf64_Rela), cmp_rela, NULL);
> + /*
> + * sort branch relocations requiring a PLT by type, symbol
> index
> + * and addend
> + */
> + nents = partition_branch_plt_relas(syms, rels, numrels,
> + sechdrs[i].sh_info);
> + if (nents)
> + sort(rels, nents, sizeof(Elf64_Rela), cmp_rela, NULL);
>
> if (!str_has_prefix(secstrings + dstsec->sh_name, ".init"))
> core_plts += count_plts(syms, rels, numrels,
> --
> 2.27.0.111.gc72c7da667-goog
>
