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
