netdev - Re: [PATCH bpf] bpf: Improve bucket

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Date:   Fri, 7 Feb 2020 10:07:32 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Martin KaFai Lau <kafai@...com>
Cc:     bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>, kernel-team@...com,
        Linux-Sparse <linux-sparse@...r.kernel.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Netdev <netdev@...r.kernel.org>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH bpf] bpf: Improve bucket_log calculation logic

On Fri, Feb 7, 2020 at 12:18 AM Martin KaFai Lau <kafai@...com> 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