Subject: Re: Optimizing instruction-cache, more packets at each stage

From: Jesper Dangaard Brouer <[email-masked]>
Date: Sun, 24 Jan 2016 15:28:14 +0100

On Thu, 21 Jan 2016 10:54:01 -0800 (PST)
David Miller <[email-masked]> wrote:

> From: Jesper Dangaard Brouer <[email-masked]>
> Date: Thu, 21 Jan 2016 12:27:30 +0100
> 
> > eth_type_trans() does two things:
> > 
> > 1) determine skb->protocol
> > 2) setup skb->pkt_type = PACKET_{BROADCAST,MULTICAST,OTHERHOST}
> > 
> > Could the HW descriptor deliver the "proto", or perhaps just some bits
> > on the most common proto's?
> > 
> > The skb->pkt_type don't need many bits.  And I bet the HW already have
> > the information.  The BROADCAST and MULTICAST indication are easy.  The
> > PACKET_OTHERHOST, can be turned around, by instead set a PACKET_HOST
> > indication, if the eth->h_dest match the devices dev->dev_addr (else a
> > SW compare is required).
> > 
> > Is that doable in hardware?  
> 
> I feel like we've had this discussion before several years ago.
> 
> I think having just the protocol value would be enough.
> 
> skb->pkt_type we could deal with by using always an accessor and
> evaluating it lazily.  Nothing needs it until we hit ip_rcv() or
> similar.

First I thought, I liked the idea delaying the eval of skb->pkt_type.

BUT then I realized, what if we take this even further.  What if we
actually use this information, for something useful, at this very
early RX stage.

The information I'm interested in, from the HW descriptor, is if this
packet is NOT for local delivery.  If so, we can send the packet on a
"fast-forward" code path.

Think about bridging packets to a guest OS.  Because we know very
early at RX (from packet HW descriptor) we might even avoid allocating
a SKB.  We could just "forward" the packet-page to the guest OS.

Taking Eric's idea, of remote CPUs, we could even send these
packet-pages to a remote CPU (e.g. where the guest OS is running),
without having touched a single cache-line in the packet-data.  I
would still bundle them up first, to amortize the (100-133ns) cost of
transferring something to another CPU.

The data-cache trick, would be to instruct prefetcher only to start
prefetching to L3 or L2, when these packet are destined for a remote
CPU.  At-least Intel CPUs have prefetch operations that specify only
L2/L3 cache.

Maybe, we need a combined solution.  Lazy eval skb->pkt_type, for
local delivery, but set the information if avail from HW desc.  And
fast page-forward don't even need a SKB.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer