linux-kernel - Re: [PATCH] bpf: Call cond_resched() to avoid soft lockup in trie

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Message-ID: <CAENh_SQgZ5yVpshKRhiezhGMDAMvgV7SmwD_8u++mACE33oNrg@mail.gmail.com>
Date: Wed, 18 Jun 2025 13:29:01 +0100
From: Matt Fleming <matt@...dmodwrite.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Song Liu <song@...nel.org>, 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>, 
	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>, 
	Matt Fleming <mfleming@...udflare.com>, hawk@...nel.org
Subject: Re: [PATCH] bpf: Call cond_resched() to avoid soft lockup in trie_free()

On Tue, Jun 17, 2025 at 4:55 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Tue, Jun 17, 2025 at 2:43 AM Matt Fleming <matt@...dmodwrite.com> wrote:
> >
>
> > soft lockup - CPU#41 stuck for 76s
>
> How many elements are in the trie that it takes 76 seconds??

We run our maps with potentially millions of entries, so it's the size
of the map plus the fact that kfree() does more work with KASAN that
triggers this for us.

> I feel the issue is different.
> It seems the trie_free() algorithm doesn't scale.
> Pls share a full reproducer.

Yes, the scalability of the algorithm is also an issue. Jesper (CC'd)
had some thoughts on this.

But regardless, it seems like a bad idea to have an unbounded loop
inside the kernel that processes user-controlled data.

Thanks,
Matt