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

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Message-ID: <CAADnVQJgOyBCCySnBkTk-VCsz0dy+ppdGHpggxbtDpBBGhaXVg@mail.gmail.com>
Date: Wed, 18 Jun 2025 07:01:13 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Matt Fleming <matt@...dmodwrite.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>, Jesper Dangaard Brouer <hawk@...nel.org>
Subject: Re: [PATCH] bpf: Call cond_resched() to avoid soft lockup in trie_free()

On Wed, Jun 18, 2025 at 5:29 AM Matt Fleming <matt@...dmodwrite.com> wrote:
>
> 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.

1M kfree should still be very fast even with kasan, lockdep, etc.
76 seconds is an algorithm problem. Address the root cause.