Subject: Re: XDP redirect measurements, gotchas and tracepoints

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 22 Aug 2017 08:37:10 +0200

On Mon, 21 Aug 2017 15:35:42 -0700
Alexei Starovoitov <[email-masked]> wrote:

> On Mon, Aug 21, 2017 at 09:25:06PM +0200, Jesper Dangaard Brouer wrote:
> > 
> > Third gotcha(3): You got this far, loaded xdp on both interfaces, and
> > notice now that (with default setup) you can RX with 14Mpps but only
> > TX with 6.9Mpps (and might have 5% idle cycles).  I debugged this via
> > perf tracepoint event xdp:xdp_redirect, and found this was due to
> > overrunning the xdp TX ring-queue size.  
> 
> we should probably fix this somehow.

Gotcha-3 (quoted above) is an interesting problem.  At first it looks
like a driver tuning problem. But it is actually an inherent property
of XDP, as there is no-queue or push-back flow control with XDP, there
is no way to handle TX queue overrun.
 My proposed solution: I want to provide a facility for userspace to
load another eBPF program (at the tracepoint xdp_redirect), which can
"see" the issue occurring.  This allows a XDP/BPF developer to
implement their own reaction/mitigation flow-control (e.g. via a map
shared with the XDP program).

> Once tx-ing netdev added to devmap we can enable xdp on it automatically?

I think you are referring to Gotcha-2 here:

  Second gotcha(2): you cannot TX out a device, unless it also have a
  xdp bpf program attached. (This is an implicit dependency, as the
  driver code need to setup XDP resources before it can ndo_xdp_xmit).

Yes, we should work on improving this situation.  Auto enabling XDP
when a netdev is added to a devmap is a good solution.  Currently this
is tied to loading an XDP bpf_prog.  Do you propose loading a dummy
bpf_prog on the netdev? (then we need to handle 1. not replacing
existing bpf_prog, 2. on take-down don't remove "later" loaded
bpf_prog).

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