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

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Date: Fri, 1 Mar 2024 19:15:36 +0000
From: Suren Baghdasaryan <surenb@...gle.com>
To: John Hubbard <jhubbard@...dia.com>
Cc: Jiri Olsa <olsajiri@...il.com>, Alan Maguire <alan.maguire@...cle.com>, 
	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, 
	Kent Overstreet <kent.overstreet@...ux.dev>, Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH] fix linux kernel BTF builds: increase max percpu
 variables by 10x

On Fri, Mar 1, 2024 at 6:33 PM John Hubbard <jhubbard@...dia.com> wrote:
>
> On 3/1/24 06:14, Jiri Olsa wrote:
> > On Thu, Feb 29, 2024 at 10:15:23AM -0800, John Hubbard wrote:
> >>> ...
> >>> Running
> >>>
> >>> bpftool btf dump file vmlinux |grep "] VAR"
> >>>
> >>
> >> $ bpftool btf dump file vmlinux |grep "] VAR" | wc -l
> >> 4852
> >>
> >> $ bpftool btf dump file vmlinux |grep "] VAR" | tail -5
> >> [136994] VAR '_alloc_tag_cntr.9' type_id=703, linkage=static
> >> [137003] VAR '_alloc_tag_cntr.5' type_id=703, linkage=static
> >> [137004] VAR '_alloc_tag_cntr.7' type_id=703, linkage=static
> >> [137005] VAR '_alloc_tag_cntr.17' type_id=703, linkage=static
> >> [137018] VAR '_alloc_tag_cntr.14' type_id=703, linkage=static
> >>
> >>> ...should give us a sense of what's going on. I only see 375 per-cpu
> >>> variables when I do this so maybe there's something
> >>> kernel-config-specific that might explain why you have so many more?
> >>
> >> Yes, as mentioned earlier, this is specifically due to the .config.
> >> The .config is a huge distro configuration that has a lot of modules
> >> enabled.
> >
> > could you share your .config? I tried with fedora .config and got 396
> > per cpu variables, I wonder where this is coming from
> >
>
> Attaching it. And based on your results, I think that Suren's Memory
> allocation profiling patchset v4 [1] may also be required, as that is what
> I was building.

Yes, that will definitely increase the number of required per-cpu
variables since it adds a per-cpu variable for each kernel allocation
to track it. I vaguely remember now that Johannes also mentioned
hitting this limit when he was using our patchset. Allocating
encoder->percpu.vars dynamically seems to be a great way to fix this
limitation.

>
> Cc: Suren and Kent. btw, I the whole reason I went down this path was that
> I recommended your patchset in order to zero in on a memory leak that a
> colleague is debugging. This patchset provides a new view of allocations
> and leaks and we have high hopes for it. :)

Thanks for trying it out John and CC'ing us on this thread!
Suren.

>
>
> [1] https://lore.kernel.org/20240221194052.927623-1-surenb@google.com
>
>
> thanks,
> --
> John Hubbard
> NVIDIA