Messages in this thread | | | Date | Tue, 16 Jun 2020 22:39:53 +0100 | From | Will Deacon <> | Subject | Re: [PATCH v1] arm64/module: Optimize module load time by optimizing PLT counting |
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On Fri, Jun 05, 2020 at 03:22:57PM -0700, Saravana Kannan 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 too. > > 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, 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.
> This commit also disables the sorting if CONFIG_RANDOMIZE_BASE is > disabled because it looks like PLTs are not allocated for > R_AARCH64_CALL26 and R_AARCH64_JUMP26 if it's disabled. > > Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org> > Signed-off-by: Saravana Kannan <saravanak@google.com> > --- > arch/arm64/kernel/module-plts.c | 37 ++++++++++++++++++++++++++++++++- > 1 file changed, 36 insertions(+), 1 deletion(-) > > 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; > +}
Does this handle A53 erratum 843419 correctly? I'm worried that we skip the ADRP PLTs there.
> + > +/* 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?
Will
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