Re: [PATCHv16 bpf-next 1/6] bpf: run devmap xdp_prog on flush instead of bulk enqueue

Toke Høiland-Jørgensen
2021-01-22T13:40:31Z

Maciej Fijalkowski <maciej.fijalkowski@intel.com> writes:

> On Fri, Jan 22, 2021 at 03:46:47PM +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:
>>
>> Using xdp_redirect_map(with a 2nd xdp_prog patch[1]) in sample/bpf and 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
>> 5.10 rc6 | xdp_redirect_map i40e->i40e | 2.0M | 9.1M | 8.0M
>> 5.10 rc6 | xdp_redirect_map i40e->veth | 1.7M | 11.0M | 9.7M
>> 5.10 rc6 + patch | xdp_redirect_map i40e->i40e | 2.0M | 9.5M | 7.5M
>> 5.10 rc6 + patch | xdp_redirect_map i40e->veth | 1.7M | 11.6M | 9.1M
>>
>> [1] https://lore.kernel.org/bpf/20210122025007.2968381-1-liuhangbin@gmail.com
>>
>> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
>> Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
>>
>> ---
>> v16:
>> a) refactor bq_xmit_all logic and remove error label
>>
>> v15:
>> a) do not use unlikely when checking bq->xdp_prog
>> b) return sent frames for dev_map_bpf_prog_run()
>>
>> v14: no update, only rebase the code
>> v13: pass in xdp_prog through __xdp_enqueue()
>> v2-v12: no this patch
>> ---
>> kernel/bpf/devmap.c | 136 ++++++++++++++++++++++++++------------------
>> 1 file changed, 81 insertions(+), 55 deletions(-)
>>
>> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
>> index f6e9c68afdd4..c24fcffbbfad 100644
>> --- a/kernel/bpf/devmap.c
>> +++ b/kernel/bpf/devmap.c
>> @@ -57,6 +57,7 @@ struct xdp_dev_bulk_queue {
>> struct list_head flush_node;
>> struct net_device *dev;
>> struct net_device *dev_rx;
>> + struct bpf_prog *xdp_prog;
>> unsigned int count;
>> };
>>
>> @@ -327,46 +328,95 @@ bool dev_map_can_have_prog(struct bpf_map *map)
>> return false;
>> }
>>
>> +static int dev_map_bpf_prog_run(struct bpf_prog *xdp_prog,
>> + struct xdp_frame **frames, int n,
>> + struct net_device *dev)
>> +{
>> + struct xdp_txq_info txq = { .dev = dev };
>> + struct xdp_buff xdp;
>> + int i, nframes = 0;
>> +
>> + for (i = 0; i < n; i++) {
>> + struct xdp_frame *xdpf = frames[i];
>> + u32 act;
>> + int err;
>> +
>> + xdp_convert_frame_to_buff(xdpf, &xdp);
>> + xdp.txq = &txq;
>> +
>> + act = bpf_prog_run_xdp(xdp_prog, &xdp);
>> + switch (act) {
>> + case XDP_PASS:
>> + err = xdp_update_frame_from_buff(&xdp, xdpf);
>> + if (unlikely(err < 0))
>> + xdp_return_frame_rx_napi(xdpf);
>> + else
>> + frames[nframes++] = xdpf;
>> + break;
>> + default:
>> + bpf_warn_invalid_xdp_action(act);
>> + fallthrough;
>> + case XDP_ABORTED:
>> + trace_xdp_exception(dev, xdp_prog, act);
>> + fallthrough;
>> + case XDP_DROP:
>> + xdp_return_frame_rx_napi(xdpf);
>> + break;
>> + }
>> + }
>> + return nframes; /* sent frames count */
>> +}
>> +
>> static void bq_xmit_all(struct xdp_dev_bulk_queue *bq, u32 flags)
>> {
>> struct net_device *dev = bq->dev;
>> - int sent = 0, drops = 0, err = 0;
>> + unsigned int cnt = bq->count;
>> + int drops = 0, err = 0;
>> + int to_sent = cnt;
>
> Hmm if I would be super picky then I'd like to have this variable as
> "to_send", as we spoke. Could you change that?
>
> With that, you can add my:
>
> Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
>
> to next revision.
>
>> + int sent = cnt;
>> int i;
>>
>> - if (unlikely(!bq->count))
>> + if (unlikely(!cnt))
>> return;
>>
>> - for (i = 0; i < bq->count; i++) {
>> + for (i = 0; i < cnt; i++) {
>> struct xdp_frame *xdpf = bq->q[i];
>>
>> prefetch(xdpf);
>> }
>>
>> - sent = dev->netdev_ops->ndo_xdp_xmit(dev, bq->count, bq->q, flags);
>> + if (bq->xdp_prog) {
>> + to_sent = dev_map_bpf_prog_run(bq->xdp_prog, bq->q, cnt, dev);
>> + if (!to_sent) {
>> + sent = 0;
>> + goto out;
>> + }
>> + drops = cnt - to_sent;
>> + }
>> +
>> + sent = dev->netdev_ops->ndo_xdp_xmit(dev, to_sent, bq->q, flags);
>> if (sent < 0) {
>> err = sent;
>> sent = 0;
>> - goto error;
>> +
>> + /* If ndo_xdp_xmit fails with an errno, no frames have been
>> + * xmit'ed and it's our responsibility to them free all.
>> + */
>> + for (i = 0; i < cnt - drops; i++) {
>> + struct xdp_frame *xdpf = bq->q[i];
>> +
>> + xdp_return_frame_rx_napi(xdpf);
>> + }
>> }
>> - drops = bq->count - sent;
>> out:
>> + drops = cnt - sent;
>> bq->count = 0;
>>
>> trace_xdp_devmap_xmit(bq->dev_rx, dev, sent, drops, err);
>> bq->dev_rx = NULL;
>> + bq->xdp_prog = NULL;
>
> One more question, do you really have to do that per each bq_xmit_all
> call? Couldn't you clear it in __dev_flush ?
>
> Or IOW - what's the rationale behind storing xdp_prog in
> xdp_dev_bulk_queue. Why can't you propagate the dst->xdp_prog and rely on
> that without that local pointer?
>
> You probably have an answer for that, so maybe include it in commit
> message.
>
> BTW same question for clearing dev_rx. To me this will be the same for all
> bq_xmit_all() calls that will happen within same napi.

I think you're right: When bq_xmit_all() is called from bq_enqueue(),
another packet will always be enqueued immediately after, so clearing
out all of those things in bq_xmit_all() is redundant. This also
includes the list_del on bq->flush_node, BTW.

And while we're getting into e micro-optimisations: In bq_enqueue() we
have two checks:

if (!bq->dev_rx)
bq->dev_rx = dev_rx;

bq->q[bq->count++] = xdpf;

if (!bq->flush_node.prev)
list_add(&bq->flush_node, flush_list);

those two if() checks can be collapsed into one, since the list and the
dev_rx field are only ever modified together. This will also be the case
for bq->xdp_prog, so putting all three under the same check in
bq_enqueue() and only clearing them in __dev_flush() would be a win, I
suppose - nice catch! :)

-Toke