Re: [RFC PATCH bpf-next 00/14] xdp_flow: Flow offload to XDP

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Toshiaki Makita <toshiaki.makita1@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <jakub.kicinski@netronome.com>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	John Fastabend <john.fastabend@gmail.com>,
	Jamal Hadi Salim <jhs@mojatatu.com>,
	Cong Wang <xiyou.wangcong@gmail.com>,
	Jiri Pirko <jiri@resnulli.us>,
	netdev@vger.kernel.org, bpf@vger.kernel.org,
	William Tu <u9012063@gmail.com>
Subject: Re: [RFC PATCH bpf-next 00/14] xdp_flow: Flow offload to XDP
Date: Tue, 13 Aug 2019 18:44:46 -0700	[thread overview]
Message-ID: <20190814014445.3dnduyrass5jycr5@ast-mbp> (raw)
In-Reply-To: <20190813120558.6151-1-toshiaki.makita1@gmail.com>

On Tue, Aug 13, 2019 at 09:05:44PM +0900, Toshiaki Makita wrote:
> This is a rough PoC for an idea to offload TC flower to XDP.
...
>  xdp_flow  TC        ovs kmod
>  --------  --------  --------
>  4.0 Mpps  1.1 Mpps  1.1 Mpps

Is xdp_flow limited to 4 Mpps due to veth or something else?

> 
> So xdp_flow drop rate is roughly 4x faster than software TC or ovs kmod.
> 
> OTOH the time to add a flow increases with xdp_flow.
> 
> ping latency of first packet when veth1 does XDP_PASS instead of DROP:
> 
>  xdp_flow  TC        ovs kmod
>  --------  --------  --------
>  25ms      12ms      0.6ms
> 
> xdp_flow does a lot of work to emulate TC behavior including UMH
> transaction and multiple bpf map update from UMH which I think increases
> the latency.

make sense, but why vanilla TC is so slow ?

> * Implementation
> 
> xdp_flow makes use of UMH to load an eBPF program for XDP, similar to
> bpfilter. The difference is that xdp_flow does not generate the eBPF
> program dynamically but a prebuilt program is embedded in UMH. This is
> mainly because flow insertion is considerably frequent. If we generate
> and load an eBPF program on each insertion of a flow, the latency of the
> first packet of ping in above test will incease, which I want to avoid.

I think UMH approach is a good fit for this.
Clearly the same algorithm can be done as kernel code or kernel module, but
bpfilter-like UMH is a safer approach.

> - patch 9
>  Add tc-offload-xdp netdev feature and hooks to call xdp_flow kmod in
>  TC flower offload code.

The hook into UMH from TC looks simple. Do you expect the same interface to be
reused from OVS ?

> * About alternative userland (ovs-vswitchd etc.) implementation
> 
> Maybe a similar logic can be implemented in ovs-vswitchd offload
> mechanism, instead of adding code to kernel. I just thought offloading
> TC is more generic and allows wider usage with direct TC command.
> 
> For example, considering that OVS inserts a flow to kernel only when
> flow miss happens in kernel, we can in advance add offloaded flows via
> tc filter to avoid flow insertion latency for certain sensitive flows.
> TC flower usage without using OVS is also possible.
> 
> Also as written above nftables can be offloaded to XDP with this
> mechanism as well.

Makes sense to me.

>   bpf, hashtab: Compare keys in long

3Mpps vs 4Mpps just from this patch ?
or combined with i40 prefech patch ?

>  drivers/net/ethernet/intel/i40e/i40e_txrx.c  |    1 +

Could you share "perf report" for just hash tab optimization
and for i40 ?
I haven't seen memcmp to be bottle neck in hash tab.
What is the the of the key?