netdev - Re: [RFC bpf-next 0/8] bpf: accelerate insn patching speed

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Message-ID: <20190716161701.mk5ye47aj2slkdjp@ast-mbp.dhcp.thefacebook.com>
Date:   Tue, 16 Jul 2019 09:17:03 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiong Wang <jiong.wang@...ronome.com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Edward Cree <ecree@...arflare.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Andrii Nakryiko <andriin@...com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        oss-drivers@...ronome.com, Yonghong Song <yhs@...com>
Subject: Re: [RFC bpf-next 0/8] bpf: accelerate insn patching speed

On Tue, Jul 16, 2019 at 09:50:25AM +0100, Jiong Wang wrote:
> 
> Let me digest a little bit and do some coding, then I will come back. Some
> issues can only shown up during in-depth coding. I kind of feel handling
> aux reference in verifier layer is the part that will still introduce some
> un-clean code.

I'm still internalizing this discussion. Only want to point out
that I think it's better to have simpler algorithm that consumes more
memory and slower than more complex algorithm that is more cpu/memory efficient.
Here we're aiming at 10x improvement anyway, so extra cpu and memory
here and there are good trade-off to make.

> >> If there is no dead insn elimination opt, then we could just adjust
> >> offsets. When there is insn deleting, I feel the logic becomes more
> >> complex. One subprog could be completely deleted or partially deleted, so
> >> I feel just recalculate the whole subprog info as a side-product is
> >> much simpler.
> >
> > What's the situation where entirety of subprog can be deleted?
> 
> Suppose you have conditional jmp_imm, true path calls one subprog, false
> path calls the other. If insn walker later found it is also true, then the
> subprog at false path won't be marked as "seen", so it is entirely deleted.
> 
> I actually thought it is in theory one subprog could be deleted entirely,
> so if we support insn deletion inside verifier, then range info like
> line_info/subprog_info needs to consider one range is deleted.

I don't think dead code elim can remove subprogs.
cfg check rejects code with dead progs.
I don't think we have a test for such 'dead prog only due to verifier walk'
situation. I wonder what happens :)