Re: [PATCH bpf] bpf: Improve bucket_log calculation logic

On Fri, Feb 7, 2020 at 12:18 AM Martin KaFai Lau [off-list ref] wrote:

It was reported that the max_t, ilog2, and roundup_pow_of_two macros have
exponential effects on the number of states in the sparse checker.

Patch looks good, but I'd like to point out that it's not just sparse.

You can see it with a simple

    make net/core/bpf_sk_storage.i
    grep 'smap->bucket_log = ' net/core/bpf_sk_storage.i | wc

and see the end result:

      1  365071 2686974

That's one line (the assignment line) that is 2,686,974 characters in length.

Now, sparse does happen to react particularly badly to that (I didn't
look to why, but I suspect it's just that evaluating all the types
that don't actually ever end up getting used ends up being much more
expensive than it should be), but I bet it's not good for gcc either.

I do think this is a good test-case for sparse. Luc, have you looked
at what it is that then makes sparse use *so* much memory for this one
line?

             Linus