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

[Jason Wang <jasowang@xxxxxxxxxx>]

On 2019/5/23 下午9:51, Toshiaki Makita wrote:
> On 19/05/23 (木) 22:29:27, Jesper Dangaard Brouer wrote:
> > On Thu, 23 May 2019 20:35:50 +0900
> > Toshiaki Makita <makita.toshiaki@xxxxxxxxxxxxx> wrote:
> >
> > > On 2019/05/23 20:25, Toke Høiland-Jørgensen wrote:
> > > > Toshiaki Makita <makita.toshiaki@xxxxxxxxxxxxx> 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 <makita.toshiaki@xxxxxxxxxxxxx>
> > > > > ---
> > > > >   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 ?
>
> Of course I can. But I thought tun has the same problem and we can 
> decrease memory footprint by sharing the same storage between devices.


For TUN and for its fast path where vhost passes a bulk of XDP frames 
(through msg_control) to us, we probably just need a temporary bulk 
array in tun_xdp_one() instead of a global one. I can post patch or 
maybe you if you're interested in this.

Thanks


> Or if other devices want to reduce queues so that we can use XDP on 
> many-cpu servers and introduce locks, we can use this storage for that 
> case as well.
>
> Still do you prefer veth-specific solution?
>
> > You could even alloc/create it on the stack of veth_poll() and send it
> > along via a pointer to veth_xdp_rcv).
>
> Toshiaki Makita