linux-kernel - Re: [PATCH] fix linux kernel BTF builds: increase max percpu variables by 10x

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Date: Wed, 28 Feb 2024 10:20:14 +0100
From: Jiri Olsa <olsajiri@...il.com>
To: John Hubbard <jhubbard@...dia.com>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>,
	Alexei Starovoitov <ast@...nel.org>, bpf@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>, dwarves@...r.kernel.org
Subject: Re: [PATCH] fix linux kernel BTF builds: increase max percpu
 variables by 10x

On Tue, Feb 27, 2024 at 07:21:42PM -0800, John Hubbard wrote:
> When building the Linux kernel with a distro .config, most or even all
> possible kernel modules are built. This adds up to 4500+ modules, and
> based on my testing, this causes the pahole utility to run out of space,
> which shows up like this (CONFIG_DEBUG_INFO_BTF=y is required in order
> to reproduce this):
> 
>   LD      .tmp_vmlinux.btf
>   BTF     .btf.vmlinux.bin.o
> Reached the limit of per-CPU variables: 4096
> ...repeated many times...
> Reached the limit of per-CPU variables: 4096
>   LD      .tmp_vmlinux.kallsyms1
>   NM      .tmp_vmlinux.kallsyms1.syms
>   KSYMS   .tmp_vmlinux.kallsyms1.S
>   AS      .tmp_vmlinux.kallsyms1.S
>   LD      .tmp_vmlinux.kallsyms2
>   NM      .tmp_vmlinux.kallsyms2.syms
>   KSYMS   .tmp_vmlinux.kallsyms2.S
>   AS      .tmp_vmlinux.kallsyms2.S
>   LD      vmlinux
>   BTFIDS  vmlinux
> libbpf: failed to find '.BTF' ELF section in vmlinux
> FAILED: load BTF from vmlinux: No data available
> make[2]: *** [scripts/Makefile.vmlinux:37: vmlinux] Error 255
> make[2]: *** Deleting file 'vmlinux'
> make[1]: *** [/kernel_work/linux-people/Makefile:1162: vmlinux] Error 2
> make: *** [Makefile:240: __sub-make] Error 2
> 
> Increasing MAX_PERCPU_VAR_CNT by 10x avoids running out of space, and
> allows the build to succeed.

do you have an actual count of percpu variables for your config?
10x seems a lot to me

this might be a workaround, but we should make encoder->percpu.vars
dynamically allocated like we do for functions

jirka

> 
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> ---
>  btf_encoder.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/btf_encoder.c b/btf_encoder.c
> index fd04008..d9f4e80 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -50,7 +50,7 @@ struct elf_function {
>  	struct btf_encoder_state state;
>  };
>  
> -#define MAX_PERCPU_VAR_CNT 4096
> +#define MAX_PERCPU_VAR_CNT 40960
>  
>  struct var_info {
>  	uint64_t    addr;
> -- 
> 2.44.0
> 
>