linux-kernel - Re: [PATCH net-next V2 1/3] tap: use build

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Date:   Wed, 16 Aug 2017 12:07:25 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>, davem@...emloft.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kubakici@...pl
Subject: Re: [PATCH net-next V2 1/3] tap: use build_skb() for small packet



On 2017年08月16日 11:59, Michael S. Tsirkin wrote:
> On Wed, Aug 16, 2017 at 11:57:51AM +0800, Jason Wang wrote:
>>
>> 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
> Yes but then you can just drop the page frag allocator since
> XDP is assumed not to care about truesize for most packets.
>

Ok, let me do some test to see the numbers between the two methods first.

Thanks