On Fri, Apr 01, 2016 at 06:04:19AM -0700, Eric Dumazet wrote: > On Fri, 2016-04-01 at 12:49 +0800, Jason Wang wrote: > > > > On 04/01/2016 10:55 AM, Eric Dumazet wrote: > > > On Fri, 2016-04-01 at 10:13 +0800, Jason Wang wrote: > > > > > > > > >> The problem is we want to support busy polling for tun. This needs > > >> napi_id to be passed to tun socket by sk_mark_napi_id() during > > >> tun_net_xmit(). But before reaching this, XPS will set sender_cpu will > > >> make us can't see correct napi_id. > > >> > > > Looks like napi_id should have precedence then ? > > > > But then when busy polling is enabled, we may still hit the issue before > > commit 2bd82484bb4c5db1d5dc983ac7c409b2782e0154? So looks like sometimes > > (e.g for tun), we need both two fields. > > You did not clearly show me the path you take where both fields would be > needed. If you expect me to do that, it wont happen. > > > > > > > > > Only forwarding should allow the field to be cleared to allow XPS to do > > > its job. > > > > > > Maybe skb_sender_cpu_clear() was removed too early (commit > > > 64d4e3431e686dc37ce388ba531c4c4e866fb141) > > > > Not sure I get you, but this will clear napi_id too. > > Only when allowed. In your case it would not be called. > > Some people do not use tun, and want to forward or cook millions of > packets per second. sk_buff size is critical. > > If busy polling gives you 5 % of performance improvement, but cost > everyone else a performance decrease, this is a serious problem. > > XPS is a sender problem, NAPI is a receiver problem. Fields should be > shared.
Right. The issue IIUC is the weird way tun behaves: it's a netdev so linux is a sender, but it has a socket in it and then linux is the receiver too. I guess we need to find a way to special-case tun somehow? -- MST
