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

[Hangbin Liu <liuhangbin@gmail.com>]

On Sat, Apr 24, 2021 at 09:01:29AM +0200, Jesper Dangaard Brouer wrote:
> > > > >> @@ -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:
> 
> 	map = READ_ONCE(ri->map);
> 	if (unlikely(map)) {
> 		WRITE_ONCE(ri->map, NULL);
> 		err = dev_map_enqueue_multi(xdp, dev, map, [...]
> 
> For XDP, performance is the single most important factor!  You say your
> performance data, there is only a slightly impact, there must be ZERO
> impact (when your added features is not in use).
> 
> You data:
>  Version          | Test                                | Generic | Native
>  5.12 rc4         | redirect_map        i40e->i40e      |    1.9M |  9.6M
>  5.12 rc4 + patch | redirect_map        i40e->i40e      |    1.9M |  9.3M
> 
> The performance difference 9.6M -> 9.3M is a slowdown of 3.36 nanosec.
> Bjørn and others have been working really hard to optimize the code and
> remove down to 1.5 nanosec overheads.  Thus, introducing 3.36 nanosec
> added overhead to the fast-path is significant.

I re-check the performance data. The data
> Version          | Test                                | Generic | Native
> 5.12 rc4         | redirect_map        i40e->i40e      |    1.9M |  9.6M
> 5.12 rc4 + patch | redirect_map        i40e->i40e      |    1.9M |  9.3M

is done on version 5.

Today I re-did the test, on version 10, with xchg() changed to
READ_ONCE/WRITE_ONCE. Here is the new data (Generic path data was omitted
as there is no change)

Version          | Test                                | Generic | Native
5.12 rc4         | redirect_map        i40e->i40e      |  9.7M
5.12 rc4         | redirect_map        i40e->veth      | 11.8M

5.12 rc4 + patch | redirect_map        i40e->i40e      |  9.6M
5.12 rc4 + patch | redirect_map        i40e->veth      | 11.6M
5.12 rc4 + patch | redirect_map multi  i40e->i40e      |  9.5M
5.12 rc4 + patch | redirect_map multi  i40e->veth      | 11.5M
5.12 rc4 + patch | redirect_map multi  i40e->mlx4+veth |  3.9M

And after add unlikely() in the check path, the new data looks like

Version          | Test                                | Native
5.12 rc4 + patch | redirect_map        i40e->i40e      |  9.6M
5.12 rc4 + patch | redirect_map        i40e->veth      | 11.7M
5.12 rc4 + patch | redirect_map multi  i40e->i40e      |  9.4M
5.12 rc4 + patch | redirect_map multi  i40e->veth      | 11.4M
5.12 rc4 + patch | redirect_map multi  i40e->mlx4+veth |  3.8M

So with unlikely(), the redirect_map is a slightly up, while redirect_map
broadcast has a little drawback. But for the total data it looks this time
there is no much gap compared with no this patch for redirect_map.

Do you think we still need the unlikely() in check path?

Thanks
Hangbin