Re: [PATCHv8 bpf-next 1/4] bpf: run devmap xdp_prog on flush instead of bulk enqueue

[Martin KaFai Lau <kafai@fb.com>]

On Tue, Apr 20, 2021 at 10:56:56PM +0200, Toke Høiland-Jørgensen wrote:
> Martin KaFai Lau <kafai@fb.com> writes:
> 
> > On Thu, Apr 15, 2021 at 09:53:17PM +0800, Hangbin Liu wrote:
> >> From: Jesper Dangaard Brouer <brouer@redhat.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:
> >> 
> >> 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.
> 
> (side note, while we're modifying the commit message, this paragraph
> should probably be moved to the end)
> 
> >> Using 10Gb i40e NIC, 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
> > Based on the discussion in v7, a summary of what still needs to be
> > addressed will be useful.
> 
> That's fair. How about we add a paragraph like this (below the one I
> just suggested above that we move to the end):
> 
> 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.
> Clearing this up will be done in a separate patch series.
lgtm