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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 16 Aug 2017 06:59:14 +0300

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.

-- 
MST