netdev - RE: [PATCH RESEND v11 1/4] bpf: run devmap xdp_prog on flush instead of bulk enqueue

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Date:   Thu, 13 May 2021 15:23:05 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Hangbin Liu <liuhangbin@...il.com>, bpf@...r.kernel.org
Cc:     netdev@...r.kernel.org,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Jiri Benc <jbenc@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>, ast@...nel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        David Ahern <dsahern@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Björn Töpel <bjorn.topel@...il.com>,
        Martin KaFai Lau <kafai@...com>,
        Hangbin Liu <liuhangbin@...il.com>
Subject: RE: [PATCH RESEND v11 1/4] bpf: run devmap xdp_prog on flush instead
 of bulk enqueue

Hangbin Liu wrote:
> From: Jesper Dangaard Brouer <brouer@...hat.com>
> 
> This changes the devmap XDP program support to run the program when the
> bulk queue is flushed instead of before the frame is enqueued. This has
> a couple of benefits:
> 
> - It "sorts" the packets by destination devmap entry, and then runs the
>   same BPF program on all the packets in sequence. This ensures that we
>   keep the XDP program and destination device properties hot in I-cache.
> 
> - It makes the multicast implementation simpler because it can just
>   enqueue packets using bq_enqueue() without having to deal with the
>   devmap program at all.
> 
> The drawback is that if the devmap program drops the packet, the enqueue
> step is redundant. However, arguably this is mostly visible in a
> micro-benchmark, and with more mixed traffic the I-cache benefit should
> win out. The performance impact of just this patch is as follows:
> 
> Using 2 10Gb i40e NIC, redirecting one to another, or into a veth interface,
> which do XDP_DROP on veth peer. With xdp_redirect_map in sample/bpf, send
> pkts via pktgen cmd:
> ./pktgen_sample03_burst_single_flow.sh -i eno1 -d $dst_ip -m $dst_mac -t 10 -s 64
> 
> There are about +/- 0.1M deviation for native testing, the performance
> improved for the base-case, but some drop back with xdp devmap prog attached.
> 
> Version          | Test                           | Generic | Native | Native + 2nd xdp_prog
> 5.12 rc4         | xdp_redirect_map   i40e->i40e  |    1.9M |   9.6M |  8.4M
> 5.12 rc4         | xdp_redirect_map   i40e->veth  |    1.7M |  11.7M |  9.8M
> 5.12 rc4 + patch | xdp_redirect_map   i40e->i40e  |    1.9M |   9.8M |  8.0M
> 5.12 rc4 + patch | xdp_redirect_map   i40e->veth  |    1.7M |  12.0M |  9.4M
> 
> When bq_xmit_all() is called from bq_enqueue(), another packet will
> always be enqueued immediately after, so clearing dev_rx, xdp_prog and
> flush_node in bq_xmit_all() is redundant. Move the clear to __dev_flush(),
> and only check them once in bq_enqueue() since they are all modified
> together.
> 
> This change also has the side effect of extending the lifetime of the
> RCU-protected xdp_prog that lives inside the devmap entries: Instead of
> just living for the duration of the XDP program invocation, the
> reference now lives all the way until the bq is flushed. This is safe
> because the bq flush happens at the end of the NAPI poll loop, so
> everything happens between a local_bh_disable()/local_bh_enable() pair.
> However, this is by no means obvious from looking at the call sites; in
> particular, some drivers have an additional rcu_read_lock() around only
> the XDP program invocation, which only confuses matters further.
> Cleaning this up will be done in a separate patch series.
> 
> Acked-by: Toke Høiland-Jørgensen <toke@...hat.com>
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> Signed-off-by: Hangbin Liu <liuhangbin@...il.com>
> 

Acked-by: John Fastabend <john.fastabend@...il.com>