linux-kernel - Re: [PATCH] kallsyms: cache bpf symbols to avoid quadratic iteration

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Message-ID: <CABWYdi2TxJb+GUqiEpgQORnf8-ErYaPJHzE13oMKsL7bL4_4nQ@mail.gmail.com>
Date: Thu, 5 Feb 2026 19:24:46 -0800
From: Ivan Babrou <ivan@...udflare.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, kernel-team <kernel-team@...udflare.com>, 
	Florian Lehner <dev@...-flo.net>
Subject: Re: [PATCH] kallsyms: cache bpf symbols to avoid quadratic iteration

On Thu, Feb 5, 2026 at 3:11 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Thu, Jan 29, 2026 at 8:50 PM Ivan Babrou via B4 Relay
> <devnull+ivan.cloudflare.com@...nel.org> wrote:
> >
> > From: Ivan Babrou <ivan@...udflare.com>
> >
> > The existing code iterates the whole list of bpf ksyms until the right
> > one is found, which means quadratic complexity on top of linked list
> > pointer chasing under rcu. This is't noticeable for most installations,
> > but when one has 10000 bpf programs loaded, things start to add up and
> > reading from `/proc/kallsyms` slows down a lot.
>
> How real is this? 10k bpf programs?

Very much real.

> >  static int get_ksymbol_bpf(struct kallsym_iter *iter)
> >  {
> > +       unsigned int index = iter->pos - iter->pos_ftrace_mod_end;
> >         int ret;
> >
> > +       if (iter->bpf_cache) {
> > +               if (index >= iter->bpf_cache_count) {
> > +                       iter->pos_bpf_end = iter->pos;
> > +                       return 0;
> > +               }
> > +
> > +               strscpy(iter->module_name, "bpf", MODULE_NAME_LEN);
> > +               iter->exported = 0;
> > +               strscpy(iter->name, iter->bpf_cache[index].name, KSYM_NAME_LEN);
> > +               iter->value = iter->bpf_cache[index].value;
> > +               iter->type = BPF_SYM_ELF_TYPE;
> > +
> > +               return 1;
> > +       }
>
> I'm sure there are other ways to speed up get_ksymbol_bpf().
> bpf_kallsyms is a list, but the same symbols are also in
> struct latch_tree_root bpf_tree
> Integrate it into get_ksymbol_bpf().
> latch_tree_find() will be surely faster than linear search in the list.
> Or go the extra mile and implement an iterator for latch_tree.

How do you feel about Florian's (cc'd) patch here?

* https://gist.github.com/florianl/41e2bce29ffde797536eef9b2e47ba92

I can take a stab at using bpf_tree like you suggested, but if you
like his patch, we can just go with that.

>
> pw-bot: cr