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:

quoted

On 2017年08月16日 11:55, Michael S. Tsirkin wrote:

quoted

On Tue, Aug 15, 2017 at 08:45:20PM -0700, Eric Dumazet wrote:

quoted

On Fri, 2017-08-11 at 19:41 +0800, Jason Wang wrote:

quoted

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