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

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Message-ID: <CAPhsuW4ie=vvDSc97pk5qH+faoKjz+b51MDYGA3shaJwNd677Q@mail.gmail.com>
Date: Mon, 16 Jun 2025 08:50:58 -0700
From: Song Liu <song@...nel.org>
To: Matt Fleming <matt@...dmodwrite.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>, 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@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...udflare.com, 
	Matt Fleming <mfleming@...udflare.com>
Subject: Re: [PATCH] bpf: Call cond_resched() to avoid soft lockup in trie_free()

On Mon, Jun 16, 2025 at 2:55 AM Matt Fleming <matt@...dmodwrite.com> wrote:
>
> From: Matt Fleming <mfleming@...udflare.com>
>
> Calls to kfree() in trie_free() can be expensive for KASAN-enabled
> kernels. This can cause soft lockup warnings when traversing large maps,

I think this could also happen to KASAN-disabled kernels, so the commit log
is a bit misleading.

>
>   watchdog: BUG: soft lockup - CPU#41 stuck for 76s! [kworker/u518:14:1158211]
>
> Avoid an unbounded loop and periodically check whether we should reschedule.
>
> Signed-off-by: Matt Fleming <mfleming@...udflare.com>

Other than that:

Acked-by: Song Liu <song@...nel.org>

> ---
>  kernel/bpf/lpm_trie.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
> index be66d7e520e0..a35619cd99f6 100644
> --- a/kernel/bpf/lpm_trie.c
> +++ b/kernel/bpf/lpm_trie.c
> @@ -646,6 +646,8 @@ static void trie_free(struct bpf_map *map)
>                         RCU_INIT_POINTER(*slot, NULL);
>                         break;
>                 }
> +
> +               cond_resched();
>         }
>
>  out:
> --
> 2.34.1
>