Subject: Re: [PATCH bpf-next 3/3] veth: Support bulk XDP_TX

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 23 May 2019 15:29:27 +0200

On Thu, 23 May 2019 20:35:50 +0900
Toshiaki Makita <[email-masked]> wrote:

> On 2019/05/23 20:25, Toke Høiland-Jørgensen wrote:
> > Toshiaki Makita <[email-masked]> writes:
> >   
> >> This improves XDP_TX performance by about 8%.
> >>
> >> Here are single core XDP_TX test results. CPU consumptions are taken
> >> from "perf report --no-child".
> >>
> >> - Before:
> >>
> >>   7.26 Mpps
> >>
> >>   _raw_spin_lock  7.83%
> >>   veth_xdp_xmit  12.23%
> >>
> >> - After:
> >>
> >>   7.84 Mpps
> >>
> >>   _raw_spin_lock  1.17%
> >>   veth_xdp_xmit   6.45%
> >>
> >> Signed-off-by: Toshiaki Makita <[email-masked]>
> >> ---
> >>  drivers/net/veth.c | 26 +++++++++++++++++++++++++-
> >>  1 file changed, 25 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> >> index 52110e5..4edc75f 100644
> >> --- a/drivers/net/veth.c
> >> +++ b/drivers/net/veth.c
> >> @@ -442,6 +442,23 @@ static int veth_xdp_xmit(struct net_device *dev, int n,
> >>  	return ret;
> >>  }
> >>  
> >> +static void veth_xdp_flush_bq(struct net_device *dev)
> >> +{
> >> +	struct xdp_tx_bulk_queue *bq = this_cpu_ptr(&xdp_tx_bq);
> >> +	int sent, i, err = 0;
> >> +
> >> +	sent = veth_xdp_xmit(dev, bq->count, bq->q, 0);  
> > 
> > Wait, veth_xdp_xmit() is just putting frames on a pointer ring. So
> > you're introducing an additional per-cpu bulk queue, only to avoid lock
> > contention around the existing pointer ring. But the pointer ring is
> > per-rq, so if you have lock contention, this means you must have
> > multiple CPUs servicing the same rq, no?  
> 
> Yes, it's possible. Not recommended though.
> 

I think the general per-cpu TX bulk queue is overkill.  There is a loop
over packets in veth_xdp_rcv(struct veth_rq *rq, budget, *status), and
the caller veth_poll() will call veth_xdp_flush(rq->dev).

Why can't you store this "temp" bulk array in struct veth_rq ?

You could even alloc/create it on the stack of veth_poll() and send it
along via a pointer to veth_xdp_rcv).

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer