netdev - Re: [PATCH bpf-next 1/3] xdp: Add bulk XDP

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Date:   Thu, 23 May 2019 20:24:46 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>
Cc:     netdev@...r.kernel.org, xdp-newbies@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next 1/3] xdp: Add bulk XDP_TX queue

On 2019/05/23 20:11, Toke Høiland-Jørgensen wrote:
> Toshiaki Makita <makita.toshiaki@....ntt.co.jp> writes:
> 
>> XDP_TX is similar to XDP_REDIRECT as it essentially redirects packets to
>> the device itself. XDP_REDIRECT has bulk transmit mechanism to avoid the
>> heavy cost of indirect call but it also reduces lock acquisition on the
>> destination device that needs locks like veth and tun.
>>
>> XDP_TX does not use indirect calls but drivers which require locks can
>> benefit from the bulk transmit for XDP_TX as well.
> 
> XDP_TX happens on the same device, so there's an implicit bulking
> happening because of the NAPI cycle. So why is an additional mechanism
> needed (in the general case)?

Not sure what the implicit bulking you mention is. XDP_TX calls
.ndo_xdp_xmit() for each packet, and it acquires a lock in veth and tun.
To avoid this, we need additional storage for bulking like devmap for
XDP_REDIRECT.

-- 
Toshiaki Makita