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
