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

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Martin KaFai Lau <kafai@fb.com>
Cc: bpf@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	David Miller <davem@davemloft.net>,
	kernel-team@fb.com, Linux-Sparse <linux-sparse@vger.kernel.org>,
	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
	Netdev <netdev@vger.kernel.org>,
	Randy Dunlap <rdunlap@infradead.org>
Subject: Re: [PATCH bpf] bpf: Improve bucket_log calculation logic
Date: Fri, 7 Feb 2020 10:07:32 -0800	[thread overview]
Message-ID: <CAHk-=wieADOQcYkehVN7meevnd3jZrq06NkmyH9GGR==2rEpuQ@mail.gmail.com> (raw)
In-Reply-To: <20200207081810.3918919-1-kafai@fb.com>

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