Messages in this thread | | | From | Saravana Kannan <> | Date | Wed, 17 Jun 2020 19:06:25 -0700 | Subject | Re: [PATCH v1] arm64/module: Optimize module load time by optimizing PLT counting |
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On Wed, Jun 17, 2020 at 7:05 AM Will Deacon <will@kernel.org> wrote: > > Hi all, > > On Wed, Jun 17, 2020 at 10:17:33AM +0200, Ard Biesheuvel wrote: > > On Tue, 16 Jun 2020 at 23:40, Will Deacon <will@kernel.org> wrote: > > > On Fri, Jun 05, 2020 at 03:22:57PM -0700, Saravana Kannan wrote: > > > > 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, the load time for one module went down from 268ms to 100ms. > > > > Another module went down from 143ms to 83ms. > > > > > > Whilst I can see that's a significant relative saving, what proportion of > > > actual boot time are we talking about here? It would be interesting to > > > know if there are bigger potential savings elsewhere. > > > > > > > Also, 'some module' vs 'some other module' doesn't really say > > anything. Please explain which modules and their sizes. > > I suspect these are all out-of-tree modules, but yes, some metadata such as > sizes, nr or relocs etc would be good to have in the commit message. > > > > > diff --git a/arch/arm64/kernel/module-plts.c b/arch/arm64/kernel/module-plts.c > > > > index 65b08a74aec6..bf5118b3b828 100644 > > > > --- a/arch/arm64/kernel/module-plts.c > > > > +++ b/arch/arm64/kernel/module-plts.c > > > > @@ -253,6 +253,36 @@ static unsigned int count_plts(Elf64_Sym *syms, Elf64_Rela *rela, int num, > > > > return ret; > > > > } > > > > > > > > +static bool rela_needs_dedup(Elf64_Rela *rela) > > > > +{ > > > > + return ELF64_R_TYPE(rela->r_info) == R_AARCH64_JUMP26 > > > > + || ELF64_R_TYPE(rela->r_info) == R_AARCH64_CALL26; > > > > +} > > > > > > > Would it help to check the section index here as well? Call/jump > > instructions within a section are never sent through a PLT entry. > > (I tried hacking this in below) > > > > Does this handle A53 erratum 843419 correctly? I'm worried that we skip > > > the ADRP PLTs there. > > > > > > > ADRP PLTs cannot be deduplicated, as they incorporate a relative jump > > back to the caller. > > Duh yes, thanks. We can't trash the link register here. > > > > > +/* Group the CALL26/JUMP26 relas toward the beginning of the array. */ > > > > +static int partition_dedup_relas(Elf64_Rela *rela, int numrels) > > > > +{ > > > > + int i = 0, j = numrels - 1; > > > > + Elf64_Rela t; > > > > + > > > > + while (i < j) { > > > > + while (rela_needs_dedup(rela + i) && i < j) > > > > + i++; > > > > + while (!rela_needs_dedup(rela + j) && i < j) > > > > + j--; > > > > + if (i < j) { > > > > + t = *(rela + j); > > > > + *(rela + j) = *(rela + i); > > > > + *(rela + i) = t; > > > > + } > > > > + } > > > > > > This is very hard to read and I think some of the 'i < j' comparisons are > > > redundant. Would it make more sense to assign a temporary rather than > > > post-inc/decrement and recheck? > > > > > > > Agreed. > > > > Also, what's wrong with [] array indexing? > > Saravana, since our stylistic objections are reasonably vague, I tried > to clean this up so you can get an idea of how I'd prefer it to look (can't > speak for Ard). I haven't tried running this, but please feel free to adapt > it. Replacement diff below. > > Will > > --->8 > > diff --git a/arch/arm64/kernel/module-plts.c b/arch/arm64/kernel/module-plts.c > index 65b08a74aec6..204290314054 100644 > --- a/arch/arm64/kernel/module-plts.c > +++ b/arch/arm64/kernel/module-plts.c > @@ -253,6 +253,39 @@ 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; > +} > + > +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]); > + else > + j--; > + } > + > + return i; > +} > + > int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs, > char *secstrings, struct module *mod) > { > @@ -290,7 +323,7 @@ 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 +333,14 @@ 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,
Thanks Will & Ard. I'll incorporate your feedback and send a v2 within a few days.
-Saravana
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