On 2018/07/02 11:54, Jason Wang wrote: > On 2018年07月02日 10:45, Toshiaki Makita wrote: >> Hi Jason, >> >> On 2018/06/29 18:30, Jason Wang wrote: >>> On 2018年06月29日 16:09, Toshiaki Makita wrote: >> ... >>>> To fix this, poll the work instead of enabling notification when >>>> busypoll is interrupted by something. IMHO signal_pending() and >>>> vhost_has_work() are kind of interruptions rather than signals to >>>> completely cancel the busypoll, so let's run busypoll after the >>>> necessary work is done. In order to avoid too long busyloop due to >>>> interruption, save the endtime in vq field and use it when reentering >>>> the work function. >>> I think we don't care long busyloop unless e.g tx can starve rx? >> I just want to keep it user-controllable. Unless memorizing it busypoll >> can run unexpectedly long. > > I think the total amount of time for busy polling is bounded. If I was > wrong, it should be a bug somewhere. Consider this kind of scenario: 0. Set 100us busypoll for example. 1. handle_tx() runs busypoll. 2. Something like zerocopy queues tx_work within 100us. 3. busypoll exits and call handle_tx() again. 4. Repeat 1-3. In this case handle_tx() does not process packets but busypoll essentially runs beyond 100us without endtime memorized. This may be just a theoretical problem, but I was worried that more code to poll tx queue can be added in the future and it becomes realistic. >>>> Performance numbers: >>>> >>>> - Bulk transfer from guest to external physical server. >>>> [Guest]->vhost_net->tap--(XDP_REDIRECT)-->i40e --(wire)--> >>>> [Server] >>> Just to confirm in this case since zerocopy is enabled, we are in fact >>> use the generic XDP datapath? >> For some reason zerocopy was not applied for most packets, so in most >> cases driver XDP was used. I was going to dig into it but not yet. > > Right, just to confirm this. This is expected. > > In tuntap, we do native XDP only for small and non zerocopy packets. See > tun_can_build_skb(). The reason is XDP may adjust packet header which is > not supported by zercopy. We can only use XDP generic for zerocopy in > this case. I think I understand when driver XDP can be used. What I'm not sure and was going to narrow down is why zerocopy is mostly not applied. -- Toshiaki Makita
