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

[John Hubbard <jhubbard@xxxxxxxxxx>]

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.

Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
---
 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