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Date: Thu, 08 Oct 2020 07:38:50 -0700 From: John Fastabend <john.fastabend@...il.com> To: Lorenzo Bianconi <lorenzo.bianconi@...hat.com>, Jesper Dangaard Brouer <brouer@...hat.com> Cc: John Fastabend <john.fastabend@...il.com>, Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org, netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org, ast@...nel.org, daniel@...earbox.net, shayagr@...zon.com, sameehj@...zon.com, dsahern@...nel.org, Eelco Chaudron <echaudro@...hat.com>, Tirthendu Sarkar <tirtha@...il.com>, Toke Høiland-Jørgensen <toke@...hat.com> Subject: Re: [PATCH v4 bpf-next 00/13] mvneta: introduce XDP multi-buffer support Lorenzo Bianconi wrote: > > On Mon, 05 Oct 2020 21:29:36 -0700 > > John Fastabend <john.fastabend@...il.com> wrote: > > > > > Lorenzo Bianconi wrote: > > > > [...] > > > > > > > > > > > > > > In general I see no reason to populate these fields before the XDP > > > > > program runs. Someone needs to convince me why having frags info before > > > > > program runs is useful. In general headers should be preserved and first > > > > > frag already included in the data pointers. If users start parsing further > > > > > they might need it, but this series doesn't provide a way to do that > > > > > so IMO without those helpers its a bit difficult to debate. > > > > > > > > We need to populate the skb_shared_info before running the xdp program in order to > > > > allow the ebpf sanbox to access this data. If we restrict the access to the first > > > > buffer only I guess we can avoid to do that but I think there is a value allowing > > > > the xdp program to access this data. > > > > > > I agree. We could also only populate the fields if the program accesses > > > the fields. > > > > Notice, a driver will not initialize/use the shared_info area unless > > there are more segments. And (we have already established) the xdp->mb > > bit is guarding BPF-prog from accessing shared_info area. > > > > > > A possible optimization can be access the shared_info only once before running > > > > the ebpf program constructing the shared_info using a struct allocated on the > > > > stack. > > > > > > Seems interesting, might be a good idea. > > > > It *might* be a good idea ("alloc" shared_info on stack), but we should > > benchmark this. The prefetch trick might be fast enough. But also > > keep in mind the performance target, as with large size frames the > > packet-per-sec we need to handle dramatically drop. > > right. I guess we need to define a workload we want to run for the > xdp multi-buff use-case (e.g. if MTU is 9K we will have ~3 frames > for each packets and # of pps will be much slower) Right. Or configuring header split which would give 2 buffers with a much smaller packet size. This would give some indication of the overhead. Then we would likely want to look at XDP_TX and XDP_REDIRECT cases. At least those would be my use cases. > > > > > > > [...] > > > > > I do think it makes sense to drop the helpers for now, and focus on how > > this new multi-buffer frame type is handled in the existing code, and do > > some benchmarking on higher speed NIC, before the BPF-helper start to > > lockdown/restrict what we can change/revert as they define UAPI. > > ack, I will drop them in v5. > > Regards, > Lorenzo > > > > > E.g. existing code that need to handle this is existing helper > > bpf_xdp_adjust_tail, which is something I have broad up before and even > > described in[1]. Lets make sure existing code works with proposed > > design, before introducing new helpers (and this makes it easier to > > revert). > > > > [1] https://github.com/xdp-project/xdp-project/blob/master/areas/core/xdp-multi-buffer01-design.org#xdp-tail-adjust > > -- > > Best regards, > > Jesper Dangaard Brouer > > MSc.CS, Principal Kernel Engineer at Red Hat > > LinkedIn: http://www.linkedin.com/in/brouer > >
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