Re: [PATCH v2] arm64/module: Optimize module load time by optimizing PLT counting

[Ard Biesheuvel <ardb@kernel.org>]

On Tue, 23 Jun 2020 at 03:27, Saravana Kannan <saravanak@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.biesheuvel@linaro.org>

[I am no longer at Linaro so please don't use my @linaro.org address]

> Signed-off-by: Saravana Kannan <saravanak@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
>