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

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Message-ID: <87v9y11k2l.fsf@toke.dk>
Date:   Thu, 23 May 2019 13:33:54 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        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

Toshiaki Makita <makita.toshiaki@....ntt.co.jp> writes:

> 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.

The bulking is in veth_poll(), where veth_xdp_flush() is only called at
the end. But see my other reply to the veth.c patch for the lock
contention issue...

-Toke