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From: Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
Subject: Re: [PATCH bpf-next 1/3] xdp: Add bulk XDP_TX queue
In-Reply-To: <2ab04d02-634e-9420-9514-e4ede08bcb10@lab.ntt.co.jp>
References: <1558609008-2590-1-git-send-email-makita.toshiaki@lab.ntt.co.jp> <1558609008-2590-2-git-send-email-makita.toshiaki@lab.ntt.co.jp> <8736l52zon.fsf@toke.dk> <2ab04d02-634e-9420-9514-e4ede08bcb10@lab.ntt.co.jp>
Date: Thu, 23 May 2019 13:33:54 +0200
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To: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>, Alexei Starovoitov <ast@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>, "David S. Miller" <davem@davemloft.net>, Jakub Kicinski <jakub.kicinski@netronome.com>, Jesper Dangaard Brouer <hawk@kernel.org>, John Fastabend <john.fastabend@gmail.com>
Cc: netdev@vger.kernel.org, xdp-newbies@vger.kernel.org, bpf@vger.kernel.org

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

> On 2019/05/23 20:11, Toke Høiland-Jørgensen wrote:
>> Toshiaki Makita <makita.toshiaki@lab.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