Toshiaki Makita <makita.toshiaki@xxxxxxxxxxxxx> writes: > On 2019/05/23 20:11, Toke Høiland-Jørgensen wrote: >> Toshiaki Makita <makita.toshiaki@xxxxxxxxxxxxx> writes: >> >>> XDP_TX is similar to XDP_REDIRECT as it essentially redirects packets to >>> the device itself. XDP_REDIRECT has bulk transmit mechanism to avoid the >>> heavy cost of indirect call but it also reduces lock acquisition on the >>> destination device that needs locks like veth and tun. >>> >>> XDP_TX does not use indirect calls but drivers which require locks can >>> benefit from the bulk transmit for XDP_TX as well. >> >> XDP_TX happens on the same device, so there's an implicit bulking >> happening because of the NAPI cycle. So why is an additional mechanism >> needed (in the general case)? > > Not sure what the implicit bulking you mention is. XDP_TX calls > .ndo_xdp_xmit() for each packet, and it acquires a lock in veth and > tun. To avoid this, we need additional storage for bulking like devmap > for XDP_REDIRECT. The bulking is in veth_poll(), where veth_xdp_flush() is only called at the end. But see my other reply to the veth.c patch for the lock contention issue... -Toke
