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Fri, 23 Apr 2021 18:09:38 -0700 (PDT) Date: Sat, 24 Apr 2021 09:09:25 +0800 From: Hangbin Liu To: Jesper Dangaard Brouer Cc: Toke =?iso-8859-1?Q?H=F8iland-J=F8rgensen?= , bpf@vger.kernel.org, netdev@vger.kernel.org, Jiri Benc , Eelco Chaudron , ast@kernel.org, Daniel Borkmann , Lorenzo Bianconi , David Ahern , Andrii Nakryiko , Alexei Starovoitov , John Fastabend , Maciej Fijalkowski , =?iso-8859-1?Q?Bj=F6rn_T=F6pel?= , Martin KaFai Lau Subject: Re: [PATCHv9 bpf-next 2/4] xdp: extend xdp_redirect_map with broadcast support Message-ID: <20210424010925.GG3465@Leo-laptop-t470s> References: <20210422071454.2023282-1-liuhangbin@gmail.com> <20210422071454.2023282-3-liuhangbin@gmail.com> <20210422185332.3199ca2e@carbon> <87a6pqfb9x.fsf@toke.dk> <20210423185429.126492d0@carbon> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20210423185429.126492d0@carbon> Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 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 wrote: > > > Jesper Dangaard Brouer writes: > > > > > On Thu, 22 Apr 2021 15:14:52 +0800 > > > Hangbin Liu 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? Thanks Hangbin