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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Sat, 24 Apr 2021 11:53:35 +0200

Jesper Dangaard Brouer <[email-masked]> writes:

> On Sat, 24 Apr 2021 09:09:25 +0800
> Hangbin Liu <[email-masked]> wrote:
>
>> On Fri, Apr 23, 2021 at 06:54:29PM +0200, Jesper Dangaard Brouer wrote:
>> > 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.  
>> 
>> Hi Jesper,
>> 
>> From the performance data, there is only a slightly impact. Do we still need
>> to block the whole patch on this? Or if you have a better solution?
>
> I'm basically just asking you to add an unlikely() annotation:

Maybe the maintainers could add this while applying, though? Or we could
fix it in a follow-up? Hangbin has been respinning this series with very
minor changes for a while now, so I can certainly emphasise with his
reluctance to keep doing this. IMO it's way past time to merge this
already... :/

-Toke