Subject: Re: [PATCH v2 bpf-next 1/2] xdp: Add tracepoint for bulk XDP_TX

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 6 Jun 2019 21:41:05 +0200

On Thu, 6 Jun 2019 20:04:20 +0900
Toshiaki Makita <[email-masked]> wrote:

> On 2019/06/05 16:59, Jesper Dangaard Brouer wrote:
> > On Wed,  5 Jun 2019 14:36:12 +0900
> > Toshiaki Makita <[email-masked]> wrote:
> >   
> >> This is introduced for admins to check what is happening on XDP_TX when
> >> bulk XDP_TX is in use, which will be first introduced in veth in next
> >> commit.  
> > 
> > Is the plan that this tracepoint 'xdp:xdp_bulk_tx' should be used by
> > all drivers?  
> 
> I guess you mean all drivers that implement similar mechanism should use 
> this? Then yes.
> (I don't think all drivers needs bulk tx mechanism though)
> 
> > (more below)
> >   
> >> Signed-off-by: Toshiaki Makita <[email-masked]>
> >> ---
> >>   include/trace/events/xdp.h | 25 +++++++++++++++++++++++++
> >>   kernel/bpf/core.c          |  1 +
> >>   2 files changed, 26 insertions(+)
> >>
> >> diff --git a/include/trace/events/xdp.h b/include/trace/events/xdp.h
> >> index e95cb86..e06ea65 100644
> >> --- a/include/trace/events/xdp.h
> >> +++ b/include/trace/events/xdp.h
> >> @@ -50,6 +50,31 @@
> >>   		  __entry->ifindex)
> >>   );
> >>   
> >> +TRACE_EVENT(xdp_bulk_tx,
> >> +
> >> +	TP_PROTO(const struct net_device *dev,
> >> +		 int sent, int drops, int err),
> >> +
> >> +	TP_ARGS(dev, sent, drops, err),
> >> +
> >> +	TP_STRUCT__entry(  
> > 
> > All other tracepoints in this file starts with:
> > 
> > 		__field(int, prog_id)
> > 		__field(u32, act)
> > or
> > 		__field(int, map_id)
> > 		__field(u32, act)
> > 
> > Could you please add those?  
> 
> So... prog_id is the problem. The program can be changed while we are 
> enqueueing packets to the bulk queue, so the prog_id at flush may be an 
> unexpected one.

Hmmm... that sounds problematic, if the XDP bpf_prog for veth can
change underneath, before the flush.  Our redirect system, depend on
things being stable until the xdp_do_flush_map() operation, as will
e.g. set per-CPU (bpf_redirect_info) map_to_flush pointer (which depend
on XDP prog), and expect it to be correct/valid.

> It can be fixed by disabling NAPI when changing XDP programs. This stops 
> packet processing while changing XDP programs, but I guess it is an 
> acceptable compromise. Having said that, I'm honestly not so eager to 
> make this change, since this will require refurbishment of one of the 
> most delicate part of veth XDP, NAPI disabling/enabling mechanism.
> 
> WDYT?

Sound like a bug, if XDP bpf_prog is not stable within the NAPI poll...

> >> +		__field(int, ifindex)
> >> +		__field(int, drops)
> >> +		__field(int, sent)
> >> +		__field(int, err)
> >> +	),  
> > 
> > The reason is that this make is easier to attach to multiple
> > tracepoints, and extract the same value.
> > 
> > Example with bpftrace oneliner:
> > 
> > $ sudo bpftrace -e 'tracepoint:xdp:xdp_* { @action[args->act] = count(); }'

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