Date:   Wed, 17 Nov 2021 18:54:55 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Ciara Loftus <ciara.loftus@...el.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, ast@...nel.org,
        daniel@...earbox.net, davem@...emloft.net, kuba@...nel.org,
        hawk@...nel.org, john.fastabend@...il.com, toke@...hat.com,
        bjorn@...nel.org, magnus.karlsson@...el.com,
        jonathan.lemon@...il.com, maciej.fijalkowski@...el.com
Subject: Re: [RFC PATCH bpf-next 0/8] XDP_REDIRECT_XSK and Batched AF_XDP Rx

On Tue, Nov 16, 2021 at 07:37:34AM +0000, Ciara Loftus wrote:
> The common case for AF_XDP sockets (xsks) is creating a single xsk on a queue for sending and 
> receiving frames as this is analogous to HW packet steering through RSS and other classification 
> methods in the NIC. AF_XDP uses the xdp redirect infrastructure to direct packets to the socket. It 
> was designed for the much more complicated case of DEVMAP xdp_redirects which directs traffic to 
> another netdev and thus potentially another driver. In the xsk redirect case, by skipping the 
> unnecessary parts of this common code we can significantly improve performance and pave the way 
> for batching in the driver. This RFC proposes one such way to simplify the infrastructure which 
> yields a 27% increase in throughput and a decrease in cycles per packet of 24 cycles [1]. The goal 
> of this RFC is to start a discussion on how best to simplify the single-socket datapath while 
> providing one method as an example.
> 
> Current approach:
> 1. XSK pointer: an xsk is created and a handle to the xsk is stored in the XSKMAP.
> 2. XDP program: bpf_redirect_map helper triggers the XSKMAP lookup which stores the result (handle 
> to the xsk) and the map type (XSKMAP) in the percpu bpf_redirect_info struct. The XDP_REDIRECT 
> action is returned.
> 3. XDP_REDIRECT handling called by the driver: the map type (XSKMAP) is read from the 
> bpf_redirect_info which selects the xsk_map_redirect path. The xsk pointer is retrieved from the
> bpf_redirect_info and the XDP descriptor is pushed to the xsk's Rx ring. The socket is added to a
> list for flushing later.
> 4. xdp_do_flush: iterate through the lists of all maps that can be used for redirect (CPUMAP, 
> DEVMAP and XSKMAP). When XSKMAP is flushed, go through all xsks that had any traffic redirected to 
> them and bump the Rx ring head pointer(s).
> 
> For the end goal of submitting the descriptor to the Rx ring and bumping the head pointer of that 
> ring, only some of these steps are needed. The rest is overhead. The bpf_redirect_map 
> infrastructure is needed for all other redirect operations, but is not necessary when redirecting 
> to a single AF_XDP socket. And similarly, flushing the list for every map type in step 4 is not 
> necessary when only one socket needs to be flushed.
> 
> Proposed approach:
> 1. XSK pointer: an xsk is created and a handle to the xsk is stored both in the XSKMAP and also the 
> netdev_rx_queue struct.
> 2. XDP program: new bpf_redirect_xsk helper returns XDP_REDIRECT_XSK.
> 3. XDP_REDIRECT_XSK handling called by the driver: the xsk pointer is retrieved from the 
> netdev_rx_queue struct and the XDP descriptor is pushed to the xsk's Rx ring.
> 4. xsk_flush: fetch the handle from the netdev_rx_queue and flush the xsk.
> 
> This fast path is triggered on XDP_REDIRECT_XSK if:
>   (i) AF_XDP socket SW Rx ring configured
>  (ii) Exactly one xsk attached to the queue
> If any of these conditions are not met, fall back to the same behavior as the original approach: 
> xdp_redirect_map. This is handled under-the-hood in the new bpf_xdp_redirect_xsk helper so the user
> does not need to be aware of these conditions.

I don't think the micro optimization for specific use case warrants addition of new apis.
Please optimize it without adding new actions and new helpers.

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