Subject: Re: [PATCHv9 bpf-next 2/4] xdp: extend xdp_redirect_map with broadcast support

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 23 Apr 2021 18:54:29 +0200

On Thu, 22 Apr 2021 20:02:18 +0200
Toke Høiland-Jørgensen <[email-masked]> wrote:

> Jesper Dangaard Brouer <[email-masked]> writes:
> 
> > On Thu, 22 Apr 2021 15:14:52 +0800
> > Hangbin Liu <[email-masked]> wrote:
> >  
> >> diff --git a/net/core/filter.c b/net/core/filter.c
> >> index cae56d08a670..afec192c3b21 100644
> >> --- a/net/core/filter.c
> >> +++ b/net/core/filter.c  
> > [...]  
> >>  int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
> >>  		    struct bpf_prog *xdp_prog)
> >>  {
> >> @@ -3933,6 +3950,7 @@ int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
> >>  	enum bpf_map_type map_type = ri->map_type;
> >>  	void *fwd = ri->tgt_value;
> >>  	u32 map_id = ri->map_id;
> >> +	struct bpf_map *map;
> >>  	int err;
> >>  
> >>  	ri->map_id = 0; /* Valid map id idr range: [1,INT_MAX[ */
> >> @@ -3942,7 +3960,12 @@ int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
> >>  	case BPF_MAP_TYPE_DEVMAP:
> >>  		fallthrough;
> >>  	case BPF_MAP_TYPE_DEVMAP_HASH:
> >> -		err = dev_map_enqueue(fwd, xdp, dev);
> >> +		map = xchg(&ri->map, NULL);  
> >
> > Hmm, this looks dangerous for performance to have on this fast-path.
> > The xchg call can be expensive, AFAIK this is an atomic operation.  
> 
> Ugh, you're right. That's my bad, I suggested replacing the
> READ_ONCE()/WRITE_ONCE() pair with the xchg() because an exchange is
> what it's doing, but I failed to consider the performance implications
> of the atomic operation. Sorry about that, Hangbin! I guess this should
> be changed to:
> 
> +		map = READ_ONCE(ri->map);
> +		if (map) {
> +			WRITE_ONCE(ri->map, NULL);
> +			err = dev_map_enqueue_multi(xdp, dev, map,
> +						    ri->flags & BPF_F_EXCLUDE_INGRESS);
> +		} else {
> +			err = dev_map_enqueue(fwd, xdp, dev);
> +		}

This is highly sensitive fast-path code, as you saw Bjørn have been
hunting nanosec in this area.  The above code implicitly have "map" as
the likely option, which I don't think it is.

> (and the same for the generic-XDP path, of course)
> 
> -Toke
> 

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