Re: [PATCH net-next 1/6] net: skbuff: don't use union for napi_id and sender_cpu

["Michael S. Tsirkin" <mst@redhat.com>]

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