Re: Using union-find in BPF verifier (was: Enhance union-find with KUnit tests and optimization improvements)

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2024-10-17 at 15:10 +0800, Shung-Hsi Yu wrote:
> Michal mentioned lib/union_find.c during a discussion. I think we may
> have a use for in BPF verifier (kernel/bpf/verifier.c) that could
> further simplify the code. Eduard (who wrote the code shown below)
> probably would have a better idea.
> 
> On Mon, Oct 07, 2024 at 06:19:10AM GMT, Tejun Heo wrote:
> > On Mon, Oct 07, 2024 at 11:28:27PM +0800, Kuan-Wei Chiu wrote:
> > > This patch series adds KUnit tests for the union-find implementation
> > > and optimizes the path compression in the uf_find() function to achieve
> > > a lower tree height and improved efficiency. Additionally, it modifies
> > > uf_union() to return a boolean value indicating whether a merge
> > > occurred, enhancing the process of calculating the number of groups in
> > > the cgroup cpuset.
> > 
> > I'm not necessarily against the patchset but this probably is becoming too
> > much polishing for something which is only used by cpuset in a pretty cold
> > path. It probably would be a good idea to concentrate on finding more use
> > cases.

Hi Shung-Hsi,

[...]

> Squinting a bit get_loop_entry() looks quite like uf_find() and
> update_loop_entry() looks quite link uf_union(). So perhaps we could get
> a straight-forward conversion here.

I'll reply tomorrow, need to sleep on it.

> ---
> 
> Another (comparatively worst) idea is to use it for tracking whether two
> register has the same content (this is currently done with struct
> bpf_reg_state.id).

Given that union-find data structure does not support element deletion
and only accumulates merged groups, for the following code:

  0: r0 = random()
  1: r1 = r0
  2: r0 = random()

It would be necessary to re-build sets of equivalent registers at
instruction (2). I'm not sure about this use case, tbh.

[...]

Thanks,
Eduard