netdev - Re: [PATCH net-next 2/3] udp: avoid a cache miss on dequeue

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Date:   Thu, 01 Jun 2017 08:58:19 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next 2/3] udp: avoid a cache miss on dequeue

On Thu, 2017-06-01 at 12:39 +0200, Paolo Abeni wrote:
> On Wed, 2017-05-31 at 10:00 -0700, Eric Dumazet wrote:
> > On Mon, 2017-05-29 at 17:27 +0200, Paolo Abeni wrote:
> > > Since UDP no more uses sk->destructor, we can clear completely
> > > the skb head state before enqueuing.
> > 
> > ...
> > 
> > > @@ -1739,6 +1740,9 @@ static int __udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
> > >  		sk_mark_napi_id_once(sk, skb);
> > >  	}
> > >  
> > > +	/* drop all pending head states; dst, nf and sk are dropped by caller */
> > > +	secpath_reset(skb);
> > > +
> > 
> > I wonder if using skb_release_head_state() would be more appropriate ?
> > 
> > Surely more descriptive and probably not more expensive since all
> > cache lines should be already hot at this point.
> 
> Thank you for reviewing this.
> 
> I would prefer not adding more code to the core, but I think we would
> need something new, like:
> 
> skb_reset_head_state()
> {
> 	skb_dst_drop(skb);
>         secpath_reset(skb);
>         nf_reset(skb);
> 	skb_orphan(skb);
> }
> 
> because elsewhere the skb could be in inconsistent state: skb->sp !=
> NULL but with its refcount is already decremented. WDYT?

I do not believe skb->sk is set anymore in UDP receive path.

If early demux sets skb->sk for a moment, skb_steal_sock() would set
skb->sk back to NULL