Re: [PATCH net-next V2 1/3] tap: use build_skb() for small packet

[Jason Wang <jasowang@redhat.com>]

On 2017年08月16日 11:55, Michael S. Tsirkin wrote:
> On Tue, Aug 15, 2017 at 08:45:20PM -0700, Eric Dumazet wrote:
>> On Fri, 2017-08-11 at 19:41 +0800, Jason Wang wrote:
>>> We use tun_alloc_skb() which calls sock_alloc_send_pskb() to allocate
>>> skb in the past. This socket based method is not suitable for high
>>> speed userspace like virtualization which usually:
>>>
>>> - ignore sk_sndbuf (INT_MAX) and expect to receive the packet as fast as
>>>    possible
>>> - don't want to be block at sendmsg()
>>>
>>> To eliminate the above overheads, this patch tries to use build_skb()
>>> for small packet. We will do this only when the following conditions
>>> are all met:
>>>
>>> - TAP instead of TUN
>>> - sk_sndbuf is INT_MAX
>>> - caller don't want to be blocked
>>> - zerocopy is not used
>>> - packet size is smaller enough to use build_skb()
>>>
>>> Pktgen from guest to host shows ~11% improvement for rx pps of tap:
>>>
>>> Before: ~1.70Mpps
>>> After : ~1.88Mpps
>>>
>>> What's more important, this makes it possible to implement XDP for tap
>>> before creating skbs.
>> Well well well.
>>
>> You do realize that tun_build_skb() is not thread safe ?
> The issue is alloc frag, isn't it?
> I guess for now we can limit this to XDP mode only, and
> just allocate full pages in that mode.
>
>

Limit this to XDP mode only does not prevent user from sending packets 
to same queue in parallel I think?

Thanks