Re: [RFC] kbuild: bpf: Do not run pahole with -j on 32bit userspace

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On 21. 08. 24, 7:32, Jiri Slaby wrote:
On 20. 08. 24, 16:33, Jiri Olsa wrote:
On Tue, Aug 20, 2024 at 10:59:50AM +0200, Jiri Slaby (SUSE) wrote:
From: Jiri Slaby <jslaby@xxxxxxx>

== WARNING ==
This is only a PoC. There are deficiencies like CROSS_COMPILE or LLVM
are completely unhandled.

The simple version is just do there:
   ifeq ($(CONFIG_64BIT,y)
but it has its own deficiencies, of course.

So any ideas, inputs?
== WARNING ==

When pahole is run with -j on 32bit userspace (32bit pahole in
particular), it randomly fails with OOM:
btf_encoder__tag_kfuncs: Failed to get ELF section(62) data: out of memory.
btf_encoder__encode: failed to tag kfuncs!

or simply SIGSEGV (failed to allocate the btf encoder).

It very depends on how many threads are created.

So do not invoke pahole with -j on 32bit.

could you share more details about your setup?

does it need to run on pure 32bit to reproduce?

armv7l builds are 32bit only.

I can't reproduce when
doing cross build and running 32 bit pahole on x86_64..

i586 is built using 64bit kernel. It is enough to have 32bit userspace.
As written in the linked bug:
https://bugzilla.suse.com/show_bug.cgi?id=1229450#c6

FWIW, steps to reproduce locally:
docker pull jirislaby/pahole_crash
docker run -it jirislaby/pahole_crash

The VM space of pahole is exhausted:
process map: https://bugzilla.suse.com/attachment.cgi?id=876821
strace of mmaps: https://bugzilla.suse.com/attachment.cgi?id=876822

You need to run with large enough -j on a fast machine. Note that this happens on build hosts even with -j4, but they are under heavy load, so parallelism of held memory is high.

From https://bugzilla.suse.com/show_bug.cgi?id=1229450#c20:
Run on 64bit:
pahole -j32 -> 4.102 GB
pahole -j16 -> 3.895 GB
pahole -j1 -> 3.706 GB

On 32bit (the same vmlinux):
pahole -j32 -> 2.870 GB (crash)
pahole -j16 -> 2.810 GB
pahole -j1 -> 2.444 GB

Look there for full massif report.

So now I think we should disable BTF generation with 32bit pahole completely. Or someone debugs it and improves debug info loading to not eat that much.

thanks,
--
js
suse labs





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