netdev - Re: [RFC PATCH v2 bpf-next 00/15] xdp

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Message-ID: <87h831cwbg.fsf@toke.dk>
Date:   Mon, 18 Nov 2019 11:28:51 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Toshiaki Makita <toshiaki.makita1@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        Pravin B Shelar <pshelar@....org>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        William Tu <u9012063@...il.com>,
        Stanislav Fomichev <sdf@...ichev.me>
Subject: Re: [RFC PATCH v2 bpf-next 00/15] xdp_flow: Flow offload to XDP

Forgot to answer this part...

>> It would probably require a bit of refactoring in the kernel data
>> structures so they can be used without being tied to an skb. David Ahern
>> did something similar for the fib. For the routing table case, that
>> resulted in a significant speedup: About 2.5x-3x the performance when
>> using it via XDP (depending on the number of routes in the table).
>
> I'm curious about how much the helper function can improve the
> performance compared to XDP programs which emulates kernel feature
> without using such helpers. 2.5x-3x sounds a bit slow as XDP to me,
> but it can be routing specific problem.

That's specific to routing; the numbers we got were roughly consistent
with the routing table lookup performance reported here:
https://vincent.bernat.ch/en/blog/2017-ipv4-route-lookup-linux

I.e., a fib lookup takes something on the order of 30-50 ns, which
eats up quite a bit of the time budget for forwarding...

-Toke