Re: [PATCH] bpf: Call cond_resched() to avoid soft lockup in trie_free()

[Alexei Starovoitov]

On Wed, Jun 18, 2025 at 5:29 AM Matt Fleming <matt@xxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Jun 17, 2025 at 4:55 PM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > On Tue, Jun 17, 2025 at 2:43 AM Matt Fleming <matt@xxxxxxxxxxxxxxxx> 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.