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Date:   Thu, 7 Dec 2023 17:02:16 -0800
From:   Charlie Jenkins <charlie@rivosinc.com>
To:     Maxim Kochetkov <fido_max@inbox.ru>
Cc:     linux-riscv@lists.infradead.org, bigunclemax@gmail.com,
        Amma Lee <lixiaoyun@binary-semi.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Conor Dooley <conor.dooley@microchip.com>,
        Andrew Jones <ajones@ventanamicro.com>,
        Jisheng Zhang <jszhang@kernel.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 1/1] riscv: optimize ELF relocation function in riscv
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References: <20230913130501.287250-1-fido_max@inbox.ru>
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On Wed, Sep 13, 2023 at 04:05:00PM +0300, Maxim Kochetkov wrote:
> The patch can optimize the running times of insmod command by modify ELF
> relocation function.
> In the 5.10 and latest kernel, when install the riscv ELF drivers which
> contains multiple symbol table items to be relocated, kernel takes a lot
> of time to execute the relocation. For example, we install a 3+MB driver
> need 180+s.
> We focus on the riscv architecture handle R_RISCV_HI20 and R_RISCV_LO20
> type items relocation function in the arch\riscv\kernel\module.c and
> find that there are two-loops in the function. If we modify the begin
> number in the second for-loops iteration, we could save significant time
> for installation. We install the same 3+MB driver could just need 2s.
> 
> Signed-off-by: Amma Lee <lixiaoyun@binary-semi.com>
> Signed-off-by: Maxim Kochetkov <fido_max@inbox.ru>
> ---
> Changes in v4:
> - use 'while' loop instead of 'for' loop to avoid code duplicate
> ---
>  arch/riscv/kernel/module.c | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/riscv/kernel/module.c b/arch/riscv/kernel/module.c
> index 7c651d55fcbd..8c9b644ebfdb 100644
> --- a/arch/riscv/kernel/module.c
> +++ b/arch/riscv/kernel/module.c
> @@ -346,6 +346,7 @@ int apply_relocate_add(Elf_Shdr *sechdrs, const char *strtab,
>  	Elf_Sym *sym;
>  	u32 *location;
>  	unsigned int i, type;
> +	unsigned int j_idx = 0;
>  	Elf_Addr v;
>  	int res;
>  
> @@ -384,9 +385,10 @@ int apply_relocate_add(Elf_Shdr *sechdrs, const char *strtab,
>  		v = sym->st_value + rel[i].r_addend;
>  
>  		if (type == R_RISCV_PCREL_LO12_I || type == R_RISCV_PCREL_LO12_S) {
> -			unsigned int j;
> +			unsigned int j = j_idx;
> +			bool found = false;
>  
> -			for (j = 0; j < sechdrs[relsec].sh_size / sizeof(*rel); j++) {
> +			do {
>  				unsigned long hi20_loc =
>  					sechdrs[sechdrs[relsec].sh_info].sh_addr
>  					+ rel[j].r_offset;
> @@ -415,16 +417,26 @@ int apply_relocate_add(Elf_Shdr *sechdrs, const char *strtab,
>  					hi20 = (offset + 0x800) & 0xfffff000;
>  					lo12 = offset - hi20;
>  					v = lo12;
> +					found = true;
>  
>  					break;
>  				}
> -			}
> -			if (j == sechdrs[relsec].sh_size / sizeof(*rel)) {
> +
> +				j++;
> +				if (j > sechdrs[relsec].sh_size / sizeof(*rel))
> +					j = 0;
Very interesting algorithm here. Assuming the hi relocation is after the
previous one seems to be a good heuristic. However I think we can do
better. In GNU ld, a hashmap of all of the hi relocations is stored and
a list of all of the lo relocations. After all of the other relocations
have been parsed, it iterates through all of the lo relocations and
looks up the associated hi relocation in the hashmap.

There is more memory overhead here but I suspect it will be faster. I
had started to mock up a hashmap implementation to see if it was faster
but decided I should mention it here first in case somebody had some
additional insight. 

- Charlie

> +
> +			} while (j_idx != j);
> +
> +			if (!found) {
>  				pr_err(
>  				  "%s: Can not find HI20 relocation information\n",
>  				  me->name);
>  				return -EINVAL;
>  			}
> +
> +			/* Record the previous j-loop end index */
> +			j_idx = j;
>  		}
>  
>  		res = handler(me, location, v);
> -- 
> 2.40.1
> 
> 
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