On Fri, 15 Jan 2016 14:32:12 +0100
Hannes Frederic Sowa [off-list ref] wrote:

On 15.01.2016 14:22, Jesper Dangaard Brouer wrote:

quoted

Given net-next is closed, we have time to discuss controversial core
changes right? ;-)

I want to do some instruction-cache level optimizations.

What do I mean by that...

The kernel network stack code path (a packet travels) is obviously
larger than the instruction-cache (icache).  Today, every packet
travel individually through the network stack, experiencing the exact
same icache misses (as the previous packet).

I imagine that we could process several packets at each stage in the
packet processing code path.  That way making better use of the
icache.

Today, we already allow NAPI net_rx_action() to process many
(e.g. up-to 64) packets in the driver RX-poll routine.  But the driver
then calls the "full" stack for every single packet (e.g. via
napi_gro_receive()) in its processing loop.  Thus, trashing the icache
for every packet.

I have a prove-of-concept patch for ixgbe, which gives me 10% speedup
on full IP forwarding.  (This patch also optimize delaying when I
touch the packet data, thus it also optimizes data-cache misses).  The
basic idea is that I delay calling ixgbe_rx_skb/napi_gro_receive, and
allow the RX loop (in ixgbe_clean_rx_irq()) to run more iterations
before "flushing" the icache (by calling the stack).


This was only at the driver level.  I also would like some API towards
the stack.  Maybe we could simple pass a skb-list?

Changing / adjusting the stack to support processing in "stages" might
be more difficult/controversial?

I once tried this up till the vlan layer and error handling got so 
complex and complicated that I stopped there. Maybe it is possible in 
some separate stages.

I've already split the driver layer into a stage.  Next I will split
GRO layer into a stage.  The GRO layer is actually quite expensive
icache-wise as it have deep calls, as the compiler cannot inline
functions due to the flexible function pointer approach.  Simply
enable/disable GRO show 10% CPU usage drop (and perf increase).

This needs redesign of a lot of stuff and while doing so I would
switch from a more stack based approach to build the stack to try out
a more iterative one (see e.g. stack space consumption problems).

The recursive nature of the rx handler (__netif_receive_skb_core/another_round)
is not necessarily bad approach for icache usage (unless rx_handler()
call indirectly flush the icache).  But as you have shown it _is_ bad for
stack space consumption.  

-- 
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